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        "5.13 Find the number of positive divisors of each of the following.",
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        "Challenge Problems",
        "7.10 How many perfect squares less than 500 are divisible by 8? (Source: MATHCOUNTS)",
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        "7.18 Define a Benson rectangular number as a positive integer \\(a\\) that, if increased and decreased by the same positive integer \\(b\\), yields two positive integers, \\(a + b\\) and \\(a - b\\), that are both perfect squares. What is the least Benson rectangular number? (Source: MATHCOUNTS)",
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        "9.18 Compute: (101012 + 10112) ÷ (1100112 + 11012) ÷ (10002 + 1002 + 102 + 12 + 12). Hints: 82",
        "9.19 Find the positive base \\( b \\) in which the equation \\( 4 \\cdot 12 = 103 \\) is valid. Hints: 141",
        "9.20 Is there any base \\( b \\) for which 3443\\(_{b}\\) is prime? If so, provide an example. If not, explain why not. Hints: 35",
        "9.21 Find the largest prime number (in decimal form) that divides the sum,",
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        "9.24 Cleo needs to determine if a 71777-digit base 12 integer is a multiple of 3. The last five digits of the integer are 71777. Find a method that helps Cleo quickly determine whether or not the enormous integer is a multiple of 3. Hints: 27",
        "9.25 The evil villain Harris Pilton wrote three secret two-digit numbers, \\( x \\), \\( y \\), and \\( z \\) on a napkin. Berris Fueler must name three numbers, \\( A \\), \\( B \\), and \\( C \\), after which Harris will announce the value of \\( Ax + By + Cz \\). If Berris can then name Harris' three secret numbers, Harris will let Berris go. Save Berris! Come up with a way to choose \\( A \\), \\( B \\), and \\( C \\) such that Berris is sure to escape Harris' evil clutches. Hints: 96",
        "9.26 Find the sum of all the natural numbers that are three-digit palindromes when expressed in base 5. Express your answer in base 5. Hints: 149"
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        "REVIEW PROBLEMS",
        "Concept: Hunting for patterns can be very helpful when solving problems or learning more about mathematics. Play around with numbers and test simple cases. See if you can establish a pattern. Then try to determine why the pattern you discovered works.",
        "Concept: Part of the power of focusing on units digits is that they can simplify problems involving many integers into just a few cases that can be easily calculated.",
        "REVIEW PROBLEMS",
        "10.19 Find the units digit of each of the following.",
        "10.20 Find the units digit of 31986 - 21986. (Source: MATHCOUNTS)",
        "10.21 Jenny has 8 stamp books that each contain 42 pages. Each page in her books contains 6 stamps. Jenny decides to reorganize her stamp books such that each page contains 10 stamps. This will give her more space to collect stamps without having to buy new books. Under her new system, Jenny fills up 4 complete books, still with 42 pages per book. Her fifth book now contains 33 pages filled with 10 stamps per page and 1 last page with the remaining stamps. How many stamps are on that last page?",
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        "10.25 Two positive integers have units digits of 2 and 3 respectively when expressed in base b. Find the units digit of their product when b equals",
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        "Concept: Modular arithmetic becomes a powerful tool when combined with an understanding of algebra.",
        "Concept: In the solution to Problem 12.17, we used a process called induction to build on a simple result to establish a more general result. The idea is that each step along the way is essentially the same. Each new congruence is a direct result of the previous congruence and the process continues endlessly.",
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        "Concept: Congruence works in modular arithmetic much the same way that equality works with regular integers (or any numbers in the regular system of real numbers). However, instead of meaning that two integers are the same, congruence means that the integers occupy the same column in a counting grid in which the number of columns is the modulus of the system."
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        "Important: Let \\(a_1\\), \\(a_2\\), \\(b_1\\), and \\(b_2\\) be integers such that",
        "Concept: Part of what makes modular arithmetic useful is that we can usually simplify calculations by working with the residues of integers. However, be aware of exceptions. It's often easiest to work with \\(-1\\) or other integers with convenient properties relating to a particular problem.",
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        "12.25 How many integers between 1000 and 3000 inclusive are congruent to 5 (mod 7)?",
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        "12.27 Four jars of jelly beans are individually repackaged into packs of 12 jelly beans. After the maximum number of packs are filled from each of these jars, the numbers of remaining jelly beans in the jars are 3, 5, 7, and 11. If the four jars had been combined before repackaging and the maximum number of packs were filled, how many jelly beans would have been left out of the 12 packs?",
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        "12.37 Suppose a and b are positive integers, neither of which is a multiple of 3. Find all possible remainders when \\(a^2 + b^2\\) is divided by 3. (Source: UNCC) Hints: 2",
        "12.38 Ryun and Zhenya play a game with a pile of 82 toothpicks. The players take turns removing 1, 2, 3, or 4 toothpicks from the pile on each turn. The player that removes the last toothpick loses. Zhenya goes first. Help her formulate a winning strategy. Hints: 37, 71",
        "12.39 A natural number \\(n\\), has a units digit of A when expressed in base 12. Find the remainder when \\(n^2\\) is divided by 6. Hints: 124",
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        "12.44 What is the size of the largest subset, \\(S\\), of \\(\\{1, 2, 3, \\ldots, 50\\}\\) such that no pair of distinct elements of \\(S\\) has a sum divisible by \\(7\\)? (Source: AMC) Hints: 81, 50",
        "12.45 The Fibonacci sequence is defined by \\(F_1 = F_2 = 1\\) and \\(F_{n+2} = F_{n+1} + F_n\\). Find the remainder when \\(F_{2006}\\) is divided by 5. Hints: 147",
        "12.46 Find all prime numbers \\(p\\) for which \\(p^2 - 1\\) is not a multiple of 24. Hints: 105",
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        "Concept: We can think of our process of finding a single linear congruence that satisfies two linear congruences as a simplification process. We solve systems of more than 2 linear congruences (of the same one variable) by continually simplifying. We combine two congruences into one as many times as we need to either show that there are no solutions or to express the solutions as a single linear congruence."
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        "14.20 Find the inverse of each of the following:",
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        "14.24 Solve the system of linear congruences:",
        "14.25 Solve the system of linear congruences:",
        "14.26 How many integers between 200 and 500 inclusive leave a remainder of 1 when divided by 7 and a remainder of 3 when divided by 4?",
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        "Definition: The property of an integer being even or odd is known as parity.",
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        "Concept: We can often use clues to gather enough information to solve a problem that has no obvious or direct solution.",
        "15.6 Summary",
        "Concept: Number sense is not just about solving problems. Number sense helps us connect mathematics in new and different ways. Sometimes this means finding quick ways to check our work. Sometimes it means noticing entirely new mathematical relationships.",
        "Important: It helps to know your primes up to 100 because they are common in practical number theory problems.",
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        "Concept: Awareness of the ways in which algebra works together with number theory (or other subjects) adds significantly to our ability to use the tools of both algebra and number theory to solve problems.",
        "Concept: Working through the long solution to a problem helps us better understand why a quick solution works and increases the range of our problem solving abilities.",
        "Concept: When the forms of numbers we are working with do not seem to give way to a practical solution, changing the form in which we express those numbers to match the problem often simplifies the problem greatly.",
        "Concept: While we may be most comfortable with a particular method of problem solving, awareness of other methods often helps us solve more difficult problems with relative ease.",
        "Concept: Parity considerations are really just a matter of working in modulo 2. Parity arguments are common enough in problem solving to give their use special attention (in addition to their own name).",
        "Concept: Don't make a problem harder than it needs to be. While experienced problem solvers have learned that hunting through lists is often an extremely slow way to solve a problem, there are exceptions. If you can narrow your search to a small list, hunting through that list might be easiest."
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        "Concept: While there may not be a direct way to compute the answers to some problems, we can often use clues to gather enough information to solve the problem.",
        "REVIEW PROBLEMS",
        "15.20 What number is one-half of one-tenth of one-fifth of one-half of one million?",
        "15.21 Find the smaller of two prime numbers whose sum is also prime.",
        "15.22 There are 24 four-digit whole numbers that use each of the four digits 2, 4, 5, and 7 exactly once. Only one of the these four-digit numbers is a multiple of another one. Which one is it? (Source: AMC)",
        "15.23 Compute 55 · 1212 - 15 · 1212.",
        "15.24 If \\(2^5 \\cdot 8^3 \\cdot 16^2 = 4^m\\), what is the value of \\(m\\)?",
        "15.25 What is the least natural number, greater than 1, that is a factor of 11000 + 1100 + 11?",
        "15.26 How many digits are in the product \\(8^{12} \\cdot 25^{8}\\)?",
        "15.27 Find the smallest positive multiple of 1999 that ends in 2006 (last four digits).",
        "15.28 How many ordered triples of three prime numbers exist for which the sum of the members of the triple is 24? (Source: MATHCOUNTS)",
        "15.29 What is the largest 4-digit number that is equal to the cube of the sum of its digits?",
        "15.30 Find the value of \\(61^3 - 3 \\cdot 61^2 + 3 \\cdot 61 - 1\\).",
        "15.31 What is the smallest whole number such that, when divided by each of 10, 9, 8, 7, ..., 2, gives a remainder of 9, 8, 7, 6, ..., 1, respectively? (Source: MATHCOUNTS)",
        "15.32 Find the greatest common divisor of 792 and 39654.",
        "15.33 The positive integers \\(A\\), \\(B\\), \\(A - B\\), and \\(A + B\\) are all prime numbers. Find the sum of these four primes. (Source: AMC)"
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        "15.34 In how many different ways can 125 be written as the sum of 3 positive perfect squares if order does not matter and the squares do not need to be distinct?",
        "15.35 What is the value of \\(\\sqrt{(27)(243)}\\)?",
        "15.36 Find the sum of the total number of digits in each of \\(2^{2006}\\) and \\(5^{2006}\\).",
        "15.37 Find the smallest positive integer that is one less than a multiple of each of 3, 5, 7, 9, and 11.",
        "15.38 Use the digits 2, 4, 5 and 7 each once to create two prime numbers. What is the smallest possible product of two prime numbers created from these digits? (Source: MATHCOUNTS)",
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        "15.40 What is the sum of all positive two-digit integers for which the sum of the digits is a multiple of 5 and the remainder is 5 when divided by 7? (Source: MATHCOUNTS)",
        "15.41 If the integer 152AB1 is a perfect square, what is the sum of the digits of its square root?",
        "15.42 Evaluate",
        "15.43 How many positive integers less than 100 leave a remainder of 1 when divided by 2 and also when divided by 3? (Source: Mandelbrot)",
        "15.44 The sum of two prime numbers is 20. What is the greatest possible product of these two primes? (Source: MATHCOUNTS)",
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        "15.46 Find the smallest four-digit multiple of 5 that has 2, 4, and 8 among its digits.",
        "15.47 Compute A divided by B where \\(A = 66666^4\\) and \\(B = 22222^4\\).",
        "15.48 Find the prime factorization of \\(5^8 - 16\\).",
        "15.49 What is the remainder when the product of the first 25 prime numbers is divided by 4? (Source: MATHCOUNTS)",
        "15.50 If \\(2^{20} - 2^{19} = 2^x\\), what is the value of \\(x\\)?",
        "15.51 There is more than one integer greater than 1 which, when divided by any integer k such that \\(2 \\leq k \\leq 11\\), has a remainder of 1. What is the difference between the two smallest such integers? (Source: AHSME)",
        "15.52 Find the greatest common divisor of 8008 and 14014.",
        "15.53 Let \\(D(a, b, c)\\) denote the number of multiples of \\(a\\) that are less than \\(c\\) and greater than \\(b\\). For example, \\(D(2, 3, 8) = 2\\) because there are two multiples of 2 between 3 and 8. What is \\(D(9^3, 9^4, 9^6)\\)? (Source: UNCC)",
        "15.54 Evaluate"
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        "15.58 Find the value of \\(\\frac{22004 + 22001}{22003 - 22000}\\).",
        "15.59 Perform the following calculation entirely in your head:",
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        "1.19 First, we determine the largest possible exponent of a perfect power between 1 and 100. We do this by noting that \\(2^m < n^m\\) for any positive integer \\(n > 2\\) and any positive integer \\(m\\). This means we can determine the largest possible power of a perfect power less than 100 by determining the largest power of 2 less than 100:",
        "1.20 This problem is only harder than other divisor problems in that any mistake could throw our count off. The integers between 1 and 9 inclusive that are divisors of 24,516 are 1, 2, 3, 4, 6, and 9, so there are \\(\\boxed{6}\\) of them. If you counted any extra or didn't include any of these, go back and check your division.",
        "1.21 Yes. \\(11111 \\div 41 = 271\\), an integer, so 11111 is divisible by 41.",
        "1.22 Yes. \\(111111 \\div 37 = 3003\\), an integer, so 111111 is divisible by 37.",
        "1.23 We check each of the smallest positive integers for divisibility of 5040 until we find a positive integer that is not a divisor of 5040."
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        "2.10 Let n be the number of students at class. Betty noticed that n has no divisors between 1 and itself, so n is prime. Wilma noticed that n + 2 is prime. This means we are looking for the smaller of two primes that differ by 2 that are between 30 and 50. The primes in that range are 31, 37, 41, 43, and 47. Since 41 and 43 differ by 2, n = \\(\\boxed{41}\\).",
        "2.11 A two-digit number whose digits are prime is made up of the digits 2, 3, 5, or 7. We check the possible numbers from the largest in this list and work backwards until we find a prime number: 77 is divisible by 7.",
        "2.12 A perfect cube is the product of three integers: \\(n \\cdot n \\cdot n = n^{3}\\). Unless n < 2, the perfect cube has a divisor between 1 and \\(n^{3}\\) (namely n) and is therefore composite. When n = 1, \\(n^{3} = 1\\), which is also not prime, so there are \\(\\boxed{0}\\) prime cubes.",
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        "2.15 Since \\(\\underline{523}\\) and \\(\\underline{717}\\) are both odd, their sum is even and therefore divisible by 2. There are no smaller \\(\\underline{2}\\), so it is the smallest prime divisor of the sum.",
        "2.16 We must find the largest prime c such that \\(a + b + c = 25\\) where a and b are also (different) \\(\\underline{1}\\)primes. One method is to check the largest possible values of c working downwards.",
        "2.17 All two-digit numbers whose digits sum to 9 are composite:",
        "2.19 So long as we can find 5 \\(\\underline{1}\\)primes between 1000 and 37² = 1369, we can check the integers after 1000 one by one to see which have no prime divisors less than 37. The Sieve of \\(\\underline{1}\\)nygard speeds up the process.",
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        "3.31 There are many ways to create each multiple of 5 out of combinations of 25's and 15's, but here are simple ones for the 10 smallest positive multiples of 5:",
        "3.32 Of the 25 primes less than 100, only 1 is even. The other 24 are odd and their sum is therefore even. Adding the even prime, 2, the sum of all 25 primes is \\(\\underline{even}\\).",
        "3.33 There are 20 positive multiples of 5 less than 101. There are 14 positive multiples of 7 less than 101. However, there are 2 positive common multiples of 5 and 7 less than 101. This means there are 18 multiples of 5 that aren't multiples of 7 and 12 multiples of 7 that are not multiples of 5 for a total of \\(18 + 12 = \\underline{30}\\).",
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        "4.26 Common multiples of 54 and 99 are multiples of \\(\\mathrm{lcm}[54, 99] = 594\\), so they can be expressed in the form \\(594n\\). The 8 smallest positive common multiples of 54 and 99 are",
        "4.27 We want to find the smallest four-digit number that is a multiple of \\(\\mathrm{lcm}[2, 3, 5, 7] = 210\\), so we need to find the smallest value of \\(n\\) such that",
        "4.28 The positive common divisors of 84 and 132 are the positive divisors of \\(\\gcd(84, 132) = 12 = 2^2 \\cdot 3^1\\). These divisors are formed by multiplying the powers of 2 from \\(2^0\\) up to \\(2^2\\) by the powers of 3 from \\(3^0\\) up to \\(3^1\\):",
        "4.29 The positive common divisors of 162 and 540 are the positive divisors of \\(\\gcd(162, 540) = 54 = 2^1 \\cdot 3^2\\). These divisors are formed by multiplying the powers of 2 from \\(2^0\\) up to \\(2^1\\) by the powers of 3 from \\(3^0\\) up to \\(3^3\\):"
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        "4.33 Positive multiples of 6 = 2¹ · 3¹ that are perfect squares are multiples of 2² · 3² = 36 that can be written in the form 36n² for each positive integer n. Positive multiples of 6 that are perfect cubes are multiples of 2³ · 3³ = 216 that can be written in the form 216n³ for each positive integer n.",
        "4.34 The product of two positive integers is equal to the product of their GCD and LCM, so",
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        "4.39 First, none of the first five friends could have been incorrect. If one of them were, and their number was n, then friend 2n would also be incorrect, but the incorrect friends were consecutively numbered, so they would have to be friends 1 and 2 and friend 1 cannot possibly have been incorrect.",
        "4.40 Since 2, 3, and 5 divide 30N but not 7, they do not divide 30N + 7. Similarly, 7 only divides 30N + 7 if 7 divides 30N, which means N must be a multiple of 7 for 7 to divide it. Since no number less than 11 divides 30N + 7 while N < 7, we only need to check when \\(30N + 7 \\geq 11^2\\). When N = 4, 30N + 7 = 127 is prime. When N = 5, 30N + 7 = 157 is prime. However, when N = \\(\\boxed{6}\\), 30N + 7 = 187 = 11 \\cdot 17 is composite."
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        "4.41 Since perfect squares that are perfect cubes have exponents in their prime factorizations that are both multiples of 2 and multiples of 3, all the exponents must be multiples of 6. Common multiples of 18 and 30 are multiples of \\(1cm\\) [18, 30] = 90 = 2¹ · 3² · 5¹. A multiple of 90 that is both a square and a cube must be in the form",
        "4.42 None of the integers is 1 because we could always remove it to reduce the sum while keeping the product the same. Now, note that the sum of two integers both greater than 1 is never greater than their product.",
        "4.43 Any common multiple of 14, 26, and 66 must be a multiple of their LCM. No common divisor of these multiples can be larger than this LCM (since it must be a divisor of the LCM). Therefore, the GCD of the common multiples of 14, 26, and 66 is the LCM of 14, 26, and 66.",
        "4.44 In general, when a proper divisor d of an integer n is large, the value of \\(\\frac{n}{a}\\), which must itself be a proper divisor of n, is small. Since 1999, 2000, and 2001 are very close in value, we are really looking for the one that has the smallest divisor greater than 1. The smallest possible divisor greater than 1 is 2, and since \\(\\frac{2000}{2} = 1000\\) is a proper divisor of \\(\\frac{2000}{2}\\), we have our answer."
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        "4.46 For a positive integer n, we know that \\(8n = 2^3 \\cdot n\\) has 3 more powers of 2 in its prime factorization. This means that there is an even power of 2 in the prime factorization of one of n and 8n and an odd power of 2 in the other. Since all the exponents in the prime factorization of a perfect square must be even, at least one of n and 8n be must non-square.",
        "4.47 The sum of the reciprocals is a fraction with denominator 60 in lowest terms. This means that 60 is a divisor of the LCM of the three consecutive integers.",
        "4.48 The number of stamps Jenna puts on each page must divide the number of stamps she puts into each book, so the largest possible number of stamps she puts on each page is \\(\\gcd(840, 1008) = \\boxed{168}\\).",
        "4.49 We use the formula for the sum of the n smallest positive integers, \\(\\frac{n(n+1)}{2}\\).",
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        "4.54 Matthew and Alex count the same number at every common multiple of 6 and 4. Every common multiple of 6 and 4 is a multiple of \\(\\lfloor \\text{cm}[6, 4] \\rfloor = 12\\). Positive multiples of 12 can be written as \\(\\lfloor 12n \\rfloor\\) for positive integers n. These are no greater than 400:",
        "4.55 Let's evaluate the three pieces of information given by the problem:"
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        "5.17 The perfect square divisors of 3240 are in the form \\(2^{2a} \\cdot 3^{2b} \\cdot 5^{2c}\\), where \\(0 \\leq a \\leq 1\\), \\(0 \\leq b \\leq 2\\), and \\(c = 0\\). This gives \\(3 \\cdot 2 \\cdot 1 = 6\\) choices for the exponents to the primes that make up the prime factorizations of the perfect square divisors of 3240."
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        "5.21 Since \\(m\\) and \\(n\\) are relatively prime, the primes in their prime factorizations are entirely distinct. This means that the exponents in the prime factorization of \\(mn\\) are simply all those in the prime factorizations of \\(m\\) and \\(n\\). Since we multiply 1 more than each of those exponents to count divisors, we have \\(t(mn) = t(m)t(n) = 12 \\cdot 10 = 120\\) positive divisors of \\(mn\\). We multiply these divisors by noting that together they make 120/2 = 60 pairs of divisors with products of \\(mn\\). The product of all the divisors is the product of these pairs, which is \\(\\boxed{mn}\\)\\(\\boxed{60}\\).",
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        "6.12 Since \\( 17^2 = 289 \\), any composite number less than 289 must be divisible by a prime number less than \\( \\sqrt{289} = 17 \\). The largest prime that is less than 17 is 13, so we use the Sieve of Eratosthenes on primes up to 13, and we see that \\(\\boxed{227}\\) and \\(\\boxed{229}\\) are prime, and that \\(\\boxed{239}\\) and \\(\\boxed{241}\\) are prime."
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        "6.13 The only prime numbers that divide 30! are less than or equal to 30. So 2, 3, 5, 7, 11, 13, 17, 19, 23, 29 are primes that divide 30!, and there are \\(\\boxed{10}\\) of these.",
        "6.14 Dividing by \\(2^4 \\cdot 3^2\\) isolates n and makes computation simple:"
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        "6.19 Every multiple of 3 less than or equal to 60 contributes at least one factor of 3 to 60! Every multiple of 3² = 9 that is less than or equal to 60 contributes one more additional factor of 3 to 60! Every multiple of 27 contributes yet one more additional extra factor of 3 to 60!",
        "6.20 It's easiest to sum the proper divisors from greatest to least. As soon as the sum exceeds the integer itself, we know the number is abundant.",
        "6.21 In an even three-digit palindrome, the first and the last digit are the same even nonzero digit (the palindrome cannot start with 0). There are 4 such digits. For each of these 4 digits, the middle digit can be any of the 10 digits from 0 to 9 inclusive. The total number of even, three-digit palindromes is 4.10 = \\(\\underline{40}\\).",
        "6.22 A five-digit palindrome has digits in the form abcba. Since the first digit cannot be 0, there are 9 choices for a. There are 10 choices for each of b and c. Each different choice of a, b, and c creates a distinct five-digit palindrome, so there are a total of 9. 10. 10 = \\(\\underline{900}\\) of them.",
        "6.23 In Problem 6.5.4, we saw that 66 is the smallest abundant palindrome. In Problem 6.20, we saw that 77 is deficient, so we test 88:"
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        "6.24 Because of the symmetric nature of the number 11, it is a divisor of many palindromes. Expanding powers of \\(x + y\\) (where \\(x = 10\\) and \\(y = 1\\)) helps us see why the first few powers of 11 are all palindromes:",
        "6.25 First, we note that since \\(n\\) has exactly 4 positive divisors that its prime factorization is either in the form \\(a^3\\) or \\(b^3 \\cdot c^3\\), where \\(a\\), \\(b\\), and \\(c\\) are all prime.",
        "6.26 Since \\(100^{100} = (10^2)^{100} = 10^{200}\\), we know that \\(100^{100}\\) expanded has 201 digits: a 1 followed by 200 0's."
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        "6.28 The only proper divisor of a prime number \\(p\\) is 1, so \\(s(p) = 1 < p\\), meaning every prime is deficient.",
        "6.29 Since \\((n + 1)! = (n + 1)n!\\) for any positive integer \\(n\\), we can relate nearby factorials without a lot of computation:",
        "6.30 Five-digit palindromes are in the form \\(abba\\), while six-digit palindromes are in the form \\(abcca\\). In either case, there are 9 possibilities for \\(a\\) (1 through 9), and 10 possibilities for each \\(b\\) and \\(c\\), for a total of \\(9 \\cdot 10 \\cdot 10 = 900\\) five-digit palindromes and 900 six-digit palindromes. Neither total is greater.",
        "6.31 First, note the formula for the sum of the first \\(n\\) positive integers:"
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        "6.32 The only prime that is a multiple of 3 is 3. This means that any pair of twin primes other than 3 and 5 must be two integers that are 1 more and 1 less than a multiple of 3. If the smaller one is 1 more than a multiple of 3, then the larger one is a multiple of 3, which is impossible. This means that a pair of twin primes greater than 3 and 5 must be in the form \\(3n-1\\) and \\(3n+1\\) for some positive integer n. Their sum is \\(6n\\), and \\(3|6n\\), so the sum of any pair of twin primes other than 3 and 5 is a multiple of 3.",
        "6.33 As we look for clues that relate the sum of two three-digit palindromes to a four-digit palindrome, we note that since the three-digit palindromes are both less than 1000, their sum is less than 2000. This means the four-digit palindrome is in the form \\(1aa1\\) for some digit a. Now, we have a better view of the addition of two three-digit palindromes, bcb and ded:"
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        "6.34 A ten-digit palindrome has digits in the form \\(abcddecdba\\). There are 9 possibilities for \\(a\\) (1 through 9), since the ten-digit number cannot start with zero, and 10 possibilities each for \\(b, c, d\\), and \\(e\\) (0 through 9). So, the total number of ten-digit palindromes is \\(9 \\cdot 10^4 = 90000\\).",
        "6.35 Since \\(8^n = (2^3)^n = 2^{3n}\\), we can solve the problem by first finding the largest power of 2 that divides 100!:",
        "6.36 \\(T(n)\\) is the number of terminal zeros of \\(n!\\), so \\(10^{T(n)}\\) | \\(n!\\). This means \\(n!\\) has at least \\(T(n)\\) factors of 2 and \\(T(n)\\) factors of 5. Since \\(n!\\) always has at least as many factors of 2 than 5, \\(n!\\) has exactly \\(T(n)\\) factors of 5.",
        "6.37 We can rewrite the given product as the quotient of two factorials:"
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        "6.38 Let \\(p^n\\) be a power of a prime. The proper divisors of \\(p^n\\) are 1, \\(p\\), \\(p^2\\), ..., \\(p^{n-1}\\), so"
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        "6.42 Since \\(12^n = 2^{2n} \\cdot 3^n\\), we are looking for the largest value of \\(n\\) such that \\(2^{2n}\\) and \\(3^n\\) are divisors of 20!",
        "6.43 It's easier to work with integers than fractions, so we multiply through by an integer that leaves us with only integers. In this case, multiplying by 7! does the trick:"
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        "7.10 Perfect squares greater than 0 have only even exponents in their prime factorizations. Integers that are divisible by 8 include 2 to at least the third power in their prime factorizations. But, since the exponents must all be even, squares that are divisible by \\(2^3 = 8\\) must also be divisible by \\(2^4 = 16\\).",
        "7.11 Let the number be AB where A is the tens digit and B is the units digit. We are given that",
        "7.12 We are trying to determine when \\(n^2 - 3n + 2\\) is prime. Since primality is based on the existence of factors between 1 and a quantity, it seems natural to look for an algebraic factorization: \\(n^2 - 3n + 2 = (n - 1)(n - 2)\\). A prime number has only one divisor greater than 1, so the smaller of the two factors n - 1 and n - 2 must be 1. Thus, \\(n - 2 = 1\\) and n = 3 is the only solution and the answer is \\(\\boxed{1}\\)."
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        "7.13 Let the three two-digit integers we are looking for be \\(ab\\), \\(cd\\), and \\(ef\\) (where \\(a\\), \\(b\\), \\(c\\), \\(d\\), \\(e\\), and \\(f\\) are the values of the digits). Since \\(3^2 + 4^2 + 5^2 = 50\\), we have",
        "7.14 An integer that is a multiple of 15, 20, and 25 must be a multiple of their LCM, which is 300. We can express such integers in the form \\(300n\\) for integer values of \\(n\\). Our goal is to count the integers in this form such that",
        "7.15 Let \\(q\\), \\(d\\), and \\(n\\) represent the numbers of quarters, dimes, and nickels in John’s jar, respectively. We express \\(d\\) and \\(n\\) in terms of \\(q\\) to reduce the number of variables we have to work with:",
        "7.16 Let’s take a look at what happens when we subtract the sum of the digits of a given two-digit number \\(AB\\) (where \\(A\\) and \\(B\\) are digits) from the number itself:"
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        "7.17 We are looking for values of n that are at least 3 (because a polygon must have at least 3 sides), such that",
        "7.18 We know that for a and b and some positive integers m and n that",
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        "7.20 For any decimal digit \\(d\\), we have",
        "7.21 It is natural for students who don't see the answer right away to explore a bit, so before we get into the solution, let's take a look at some examples:",
        "3.5 = 15 = 42 - 1",
        "5.7 = 35 = 62 - 1",
        "11.13 = 143 = 122 - 1",
        "17.19 = 323 = 182 - 1",
        "29.31 = 899 = 302 - 1",
        "7.22 We apply the Division Theorem to this problem. We are told that when \\(x\\) is divided by \\(y\\) that the quotient is \\(u\\) and the remainder is \\(v\\):",
        "7.23 A perfect square that is a multiple of both 3 and 4 must have exponents of at least 2 for prime factors 2 and 3, so it must be a multiple of \\(2^2 \\cdot 3^2 = 36\\). These integers can be written in the form \\(36n^2\\) for integers \\(n\\). Since we are looking for three-digit integers, we have the inequality"
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        "7.24 The first thing we do is change the word problem into a math problem. Let \\(A\\) be the number of pigs the first farmer gives to the second and let \\(B\\) denote the number of goats the first farmer gives to the second. Let \\(n\\) be the total amount of debt settled in the transaction. Note that either \\(A\\) or \\(B\\) can be a negative integer. This would represent the first farmer receiving animals. Our equation is"
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        "7.26 We are given several pieces of information to work with and we’ll have to combine them. We are looking for even integers, so they must have positive powers of 2 in their prime factorizations. The numbers are also squares and cubes, so the exponents in their prime factorizations must be multiples of 2 and 3, and therefore 6. We are looking for integers in the form \\(2^6 n^6 = 64n^6\\) that have four digits:",
        "7.27 This one turns out to be less difficult than it initially looks. Since we are dealing with products of integers, we take a second look at the equation where the right-hand side is prime factored:",
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        "8.23 We are looking for the smallest base \\(b\\) such that \\(100_b \\leq 62 < 1000_b\\), which is the same as saying that \\(b^2 \\leq 62 < b^3\\). The smallest perfect cube greater than 62 is 64, so the smallest possible value of \\(b\\) is \\(\\sqrt[3]{64} = \\boxed{4}\\).",
        "Challenge Problems",
        "8.24 The number of terminal zeros in which an integer ends in base 12 is the number of factors of 12 that divide that integer. Since \\(12 = 2^2 \\cdot 3^1\\), we must find the number of factors of both 2 and 3 in 10! in order to find the number of factors of 12.",
        "8.25 The goal is to count in base 3 using only binary digits. The \\(100^th\\) smallest positive binary integer is \\(100 = 1100100_2\\), so the \\(100^th\\) smallest positive integer that can be written with only the binary digits is \\(1100100_3 = \\boxed{981}\\).",
        "8.26 It's easier to count the integers that don't have a 7 digit than it is to count those that do, so we use complementary counting to solve this problem.",
        "8.27 The smallest positive integer that needs 4 digits when expressed in base \\(b\\) is \\(1000_b = b^3\\). Therefore, the largest integer that can be expressed in base \\(b\\) using 3 digits is \\(b^3 - 1\\).",
        "8.28 Since \\(15 = 3^1 \\cdot 5^1\\), the largest possible value of \\(n\\) for which \\(15^n = 942!\\) is the largest possible value of \\(n\\) for which both \\(3^n = 942!\\) and \\(5^n = 942!\\). Since \\(942!\\) has many more factors of 3 than it does 5, our answer will be the number of factors of 5 in \\(942!\\)."
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        "8.29 The answer is \\(\\underline{\\text{no}}\\).",
        "8.30 Since \\(3^{2} = 9\\), we can convert directly from base 3 to base 9 by converting each pair of digits (starting with the rightmost digits) to base 9. Since \\(x = 121122111222111122223\\), the leftmost digit in base 9 is the conversion of the leftmost digit pair from base 3, to base 9. Since \\(12_{3} = 5_{9}\\), the answer is \\(\\underline{\\text{5}}\\).",
        "8.31 Since \\(23_{b} = 2b + 3\\) and \\(b > 3\\), \\(23_{b}\\) can be any odd integer greater than 2(3) + 3 = 9. We are looking for the next smallest odd perfect square, which is \\(5^{2} = 25\\). Since \\(2b + 3 = 25\\), \\(b = \\underline{\\text{11}}\\) is our answer.",
        "8.32 An eight-bit binary word is a nonnegative integer with less than 9 digits when expressed in binary. This means we are counting the integers from 0 to \\(2^{8} - 1\\) inclusive, of which there are \\(2^{8} = \\underline{\\text{256}}\\). We could also view each digit as a \"choice\". There are 2 choices for each of 8 digits, so \\(2^{8}\\) eight-bit binary words are possible.",
        "8.33 We can count 1 down from 32768 = 100000\\(_{8}\\) to get \\(\\underline{\\text{777778}}\\).",
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        "8.36 Since \\(47_{a} = 74_{b}\\), we know that \\(4a + 7 = 7b + 4\\) where a and b are each at least 8 (since the largest digit of each base number is 7). Since \\(b \\geq 8\\), the smallest possible value of the integer represented as \\(47_{a}\\) and \\(74_{b}\\) is \\(7 \\cdot 8 + 4 = 60\\). However, \\(4a + 7 = 4(a + 1) + 3\\), so we are looking for an integer that leaves a remainder of 3 when divided by 4. We try b = 9 and get 7b + 4 = 67. Since \\(4a + 7 = 67\\) has a solution a = 15, we have a pair of bases that work.",
        "8.37 \\(343 = 7^{3} = 1000_{7}\\), so the first 343 positive integers in base 7 are \\(17, 27, \\ldots 1000_{7}\\). Any number in this list that neither includes 4 or 5 only includes the digits 0, 1, 2, 3, and 6. If we replace 6 with 4, these have the same decimal expansions as the integers in base 5. Since there are \\(5^{3} = 125\\) positive integers less than or equal to 1000, there are 125 integers less than or equal to 1000; that contain no 4's or 5's in base 7, which means there are \\(343 - 125 = 218\\) integers that include a 4 or a 5."
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        "8.39 Let a, b, and c be digits such that \\(N = abc_7 = cba_9\\). This gives us the equation",
        "8.40 We can use binomial expansion to more easily convert each power of \\(10b_{+1}\\) to each given base b:",
        "8.41 The integers that can only be written using 0, 1, 2, 3, 5, 6, 7, 8, 9 in base 10 correspond with the integers that are written with 0, 1, 2, 3, 4, 5, 6, 7, 8 in base 9, where 5 in base 10 corresponds with 4 in base 9, 6 in base 10 corresponds with 5 in base 9, etc. So 2005 on the odometer is the same as 2004, which is \\(\\boxed{1462}\\) miles in base 10.",
        "8.42 Since the integer has 2 terminal zeros in base 3, it is a multiple of \\(3^2\\). Since it has a terminal zero in each base 4 and base 5, it is a multiple of each 4 and 5. This means the integer is a multiple is \\(lcm[9, 4, 5] = 2^2 \\cdot 3^2 \\cdot 5^1 = 180\\). If the integer has a terminal zero in base b, then it is a multiple of b. Since the integer must have \\(t(180) = 18\\) positive divisors (possibly others), there are \\(18 - 4 = \\boxed{14}\\) other bases (other than 1, 3, 4, and 5) in which the integer must have at least one terminal zero.",
        "8.43 Since \\(VYZ\\), \\(VYX\\), and \\(VWW\\) are three consecutive base-5 integers, their units digits must be 3, 4, and 0 respectively because their middle digit changed between the second and third of these numbers. This means that \\(Z = 3\\), \\(X = 4\\), and \\(W = 0\\) and the three consecutive integers are \\(VY3\\), \\(VY4\\), and \\(VVO\\). Additionally, \\(V = Y + 1\\), because when the middle digit went up, it couldn't have gone up from 4 to 0 because those digits are already used by other letters. This means \\(V = 2\\) and \\(Y = 1\\), so \\(XYZ_5 = 413_5 = \\boxed{108}\\).",
        "8.44 Base-3 integers that don't use the digit 2 correspond to binary integers. We need to count all the integers up to 1992 that have a base-3 representation that looks like binary, so we start by converting 1992 to base 3: \\(1992 = 2201210_3\\). The largest integer less than this that has only digits 0 and 1 is \\(1111111_3\\). This corresponds to the binary integer that is the last we need to count: \\(1111111_2 = 2^7 - 1 = \\boxed{127}\\)."
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        "9.16 All of these problems can be solved by converting the integers to base 10 and using regular division or even using base number division. However, we look for methods that can be applied more easily once they are fully understood. Make sure you see why each method works!"
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        "9.18 This problem seems at first like a bigger headache than it really is. However, starting patiently by adding the binary numbers in the parentheses quickly makes the job easier:",
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        "9.20 We know \\(b > 4\\) since the digit 4 appears. For every \\(b > 4\\), it's true that \\(11_b \\cdot 313_b = 3443_b\\). There are a number of ways in which we can see why this is true. Straightforward multiplication requires no carrying and long division reveals a similar result.",
        "9.21 Note that",
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        "9.23 We are given that \\(12_b \\cdot 15_b \\cdot 16_b = 3146_b\\). We can express each integer in terms of base-b digit bundles, giving us a polynomial equation:",
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        "10.20 The units digit of powers of 2 and 3 both repeat in cycles four long. Since \\(1986 \\div 4\\) has a remainder of 2, the units digit of \\(3^{1986} - 2^{1986}\\) is the same as \\(3^{2} - 2^{2} = \\boxed{5}\\).",
        "10.21 Since Jenny's new system of arranging stamps includes 10 on each page, the number of the last page will be the units digit of her total number of stamps (in base 10). That units digit is the same as the units digit of \\(8 \\cdot 2 \\cdot 6 = 96\\), which is \\(\\boxed{6}\\).",
        "10.22 Since the product is positive, we need only check the possible odd units digits and we find that \\(\\boxed{7}\\) is the only one that works: \\(7 \\cdot 7 = 49\\).",
        "10.23 First, we note that the product is odd, so neither of the primes is 2. Also, since the units digit of the product is neither 0 nor 5, the product is not a multiple of 5. Since all integers with units digits of 5 are multiples of 5, we don't need to worry about 5 as a possible units digit. We test the remaining cases. We need only test 10 cases since we can organize the products with the smaller units digit first:"
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        "0.1 = 0  5.6 = 30",
        "1.2 = 2  6.7 = 42",
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        "3.4 = 12  8.9 = 72",
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        "10.30 Since \\(n^4 = (n^2)^2\\), we are looking for the possible base-5 units digits of squares of squares. From Problem 10.26, we know that squares have base-5 units digits of 0, 1, or 4. We also know that the squares of such integers have base-5 units digits of 0, 1, or 1 respectively, so \\(\\underline{0}\\) and \\(\\underline{1}\\) are the only possible units digits of perfect fourth powers in base 5. Notice that this means perfect fourth powers leave remainders of only 0 or 1 when divided by 5.",
        "10.32 The units digit of \\((133^{13})^{3}\\) is the same as the units digit of \\((3^{13})^{3} = 3^{39}\\). As we've seen, the units digits of powers of three repeat in a cycle of 4. Since \\(39 = 4 \\cdot 9 + 3\\), the units digit of \\(3^{39}\\) is the same as that of \\(3^{8} = 27\\), which is \\(\\underline{7}\\).",
        "10.33 The pages are consecutive integers and in Problem 10.28 we saw the possible units digits of products of consecutive integers. We sum the units digits of the consecutive integers in both cases where the product has a units digit of 6: \\(2 + 3 = 5\\) and \\(7 + 8 = 15\\) have the same units digit, \\(\\underline{5}\\).",
        "10.34 As we've seen already, the units digits of powers of 2 repeat in a cycle of 4. Since there are powers of 2 with units digit 6, we know that \\(100 \\div 4 = \\underline{25}\\) of the given powers of 2 have units digits of 6.",
        "10.35 The units digit of \\(mn\\) is \\(16 = 1\\). Searching for a units digit for \\(n\\) (which is clearly odd), we find that \\(7 \\cdot 3 = 1\\), so \\(\\underline{3}\\) is the units digit of \\(n\\).",
        "10.36 First we note that the units digit of \\(m^2n = 39^{39}\\) is the same as the units digit of \\(9^{39}\\). The units digits of powers of 9 repeat in a cycle of 2, so this is the same as the units digit of \\(9^1 = 9\\). Since the units digit of \\(n\\) is 1, we know the units digit of \\(m^2\\) is 9. Searching through the possibilities, we know that the units digit of \\(m\\) is either \\(\\underline{3}\\) or \\(\\underline{7}\\)."
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        "10.38 An integer that is a multiple of both 4 and 6 is a multiple of \\(lcm[4, 6] = 12\\). Our goal is to count the three-digit multiples of 12 with units digits of 2. For a positive integer \\(n\\), \\(12n\\) has a units digit of 2 when \\(n\\) has a units digit of 1 or 6. In other words, \\(n\\) is 1 more than a multiple of 5. We can rewrite \\(n\\) as \\(5m + 1\\) for nonnegative integers \\(m\\).",
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        "10.41 Cubing each units digit, we find that a cube has a units digit of 3 only when the units digit of its cube root has a units digit of 7, so B = 7. Also, we note that",
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        "10.44 Let N be the integer we seek. We know that N is 2 more than a multiple of each of 3, 4, 5, and 6, so N - 2 is a multiple of \\(\\lfloor \\text{cm} \\rfloor 3\\), 4, 5, 6] = 60. Since N > 2, the number we seek is N = 62.",
        "10.45 The very first thing we notice is that b ≥ 4 because we are using digits up to 3 already. Our sum is really a product of 7 and 32b, so we know that 7 · 2 = 14 has a units digit of 2 in base b. This means 14 = nb + 2 for some integer n and nb = 12 means that b | 12. The divisors of 12 greater than 3 are 4, 6, and 12, and all 3 satisfy the problem statement.",
        "10.46 Mersenne primes are \\(\\frac{2}{3} \\times 2^p - 1\\) for prime values of p. If a particular Mersenne prime has a unit’s digit of 3, then 2^p has a unit’s digit of 4. However, in the repeating cycle of units digits of powers of 2, only the powers of 2 in the form \\(2^{4n+2}\\) for some nonnegative integer n have units digits of 4. Since \\(4n + 2 = 2(n + 1) = p\\), 2 | p, so p = 2. This proves there is only 1 Mersenne prime with a unit’s digit of 3, namely, \\(2^2 - 1 = 3\\)."
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        "11.19 We don't need to convert a single fraction. We just count the ones with prime factorizations that include any prime other than 2 or 5: 3, 6, 7, and 9, so \\(\\frac{4}{4}\\) of them.",
        "11.20 We expand the factorials and then convert to decimal form:",
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        "11.26 First, we find the repeating decimal expansion of our fraction: \\(\\frac{1}{17} = 0.0588235294117647\\). We want the \\(4037^\\text{th}\\) digit of a 16-digit block of digits. The remainder from \\(4037 \\div 16\\) is 5, so our answer is the \\(5^\\text{th}\\) digit of the block, which is \\(\\boxed{2}\\).",
        "11.27 The first term is 1/100 and the common ratio is 7/100, so",
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        "12.26 If \\( n \\equiv -n \\pmod{12} \\), then \\( n - (-n) = 12k \\) for some integer \\( k \\).",
        "12.27 Let the number of jelly beans in the individual jars be \\( a \\), \\( b \\), \\( c \\), and \\( d \\). The modulo-12 residues of \\( a \\), \\( b \\), \\( c \\), and \\( d \\) are 3, 5, 7, and 11. So,",
        "12.29 Multiplying \\( a \\equiv 19 \\pmod{30} \\) by 3, we get \\( 3a \\equiv 57 \\equiv 27 \\pmod{30} \\). This means that",
        "12.30 It takes a while to find that \\( 7^{10} \\equiv 1 \\pmod{11} \\). We use it to break down the problem:",
        "12.31 Note that \\( 9 \\equiv 2 \\pmod{7} \\) and \\( 5 \\equiv -2 \\pmod{7} \\):",
        "12.32 Squaring each of the modulo-6 residues, we find that perfect squares can only have modulo-6 residues of 0, 1, 3, and 4. As Susie already knows, this means that no perfect square leaves a remainder of 2 when divided by 6."
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        "12.35 The difference between 617n and 943n is a multiple of 18, so",
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        "12.39 From the units digit of n, we know that \\(n \\equiv 10 \\pmod{6}\\). Since 6 | 12, we also know that \\(n \\equiv 10 \\equiv 4 \\pmod{6}\\). Squaring, we get \\(n^2 \\equiv 4^2 \\equiv 16 \\equiv 4 \\pmod{6}\\).",
        "12.40 Since \\(69 \\equiv 90 \\equiv 125 \\pmod{N}\\), we know that both of the following are integers:",
        "12.41 Let n be the positive integer in question.",
        "12.42 This problem looks rather daunting. Without a clear direction, we might as well work out the remainder when the first term is divided by 7. If we are lucky, we’ll notice something along the way that will help us.",
        "12.43 We prove more generally that for k people, the number of people who shook hands with an odd number of others must be even.",
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        "12.47 The pigeonhole principle tells us that among any 51 integers, there are 2 with the same modulo-50 residue. Each of these integers can be written as \\( 50n + r \\) where \\( n \\) is an integer and \\( r \\) is the shared residue. Note that",
        "12.48 When applying modular arithmetic to difficult number theory problems, the difficult part is often coming up with the right modulus to use. The number of possible residues of squares in modulus 3, 4, and 8 are limited, which is often helpful for solving problems. In this case, modulo 4 can help us."
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        "13.11 An integer is a multiple of 9 if and only if the sum of its digits is a multiple of 9.",
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        "13.13 An integer is a multiple of 8 if and only if its last three digits form a multiple of 8.",
        "13.14 Since 72 = 8 \\(\\cdot\\) 9 and gcd(8, 9) = 1, we use the divisibility rules for 8 and 9 to find the digits A and B.",
        "13.15 Since 45 = 5 \\(\\cdot\\) 9 and gcd(5, 9) = 1, we use the divisibility rules for 5 and 9 to find the digits A and B.",
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        "13.16 We know the digits of the integer, so we think to apply any rule that comes to mind that uses all the digits, regardless of their order. Hence, we apply the divisibility rule for 9 and note that the nine-digit number is congruent to"
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        "13.17 An integer is a multiple of 4 if and only if its last two digits are a multiple of 4. Also, the units digit must be even in order for an integer to be even, so this gives us a good starting point for testing cases.",
        "13.18 A four-digit palindrome must be in the form ABBA for some digits A and B. Such a palindrome is a multiple of 9 if and only if the sum of its digits is a multiple of 9. In other words, \\(2(A + B)\\) must be a multiple of 9. Since 2 and 9 are relatively prime, \\(A + B\\) must be a multiple of 9.",
        "13.19 Since \\(99 = 9 \\cdot 11\\), and \\(gcd(9, 11) = 1\\), there is a temptation to try to build a divisibility rule by combining those for 9 and 11. However, noting that \\(100 = 10^2 \\equiv 1 \\pmod{99}\\) gives us a nicer possibility."
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        "13.20 Let's get a grip on the arithmetic behind the divisibility rule. For any given integer n, we can rewrite n as \\(10A + B\\), where B is the units digit of n. Now, let's be very precise about writing the meaning behind the divisibility rule mathematically. We are told that",
        "13.21 There are lots of three-digit positive integers with digit sums of 13, and they aren't easily described in a useful way. So, we'll start with the fact that the integers we're counting are congruent to 1 (mod 4). We want integers whose last two digits are congruent to 1 (mod 4). Such integers end in 01, 05, 09, 13, 17, ..., 97. Each of these 100/4 = 25 pairs of digits can be associated with at most one hundreds digit such that the sum of all three digits is 13. Since the hundreds digit is between 1 and 9 inclusive, we want the pairs of last two digits to have a sum between 4 and 12 inclusive. We throw out 01, 21, 49, 69, 77, 85, 89, and 97. This leaves 25 - 8 = 17 pairs of last two digits that can each be grouped with a hundreds digit to make a digit sum of 13.",
        "13.22 We are looking for an integer that is a multiple of lcm[7, 8, 9] = 504. Let our six-digit number be 523ABC for some digits A, B, and C. First we note that 523ABC is a multiple of 504 if and only if 523ABC - 504000 = 19ABC is a multiple of 504. Now, we need to find an integer 19ABC that is a multiple of 504. We could apply divisibility rules to the problem, but we can also simply go hunting! What makes hunting easier is the fact that 19000/504 is approximately equal to 19000/500 = 38. We note that 38 \\(\\cdot\\) 504 = 19152 and 39 \\(\\cdot\\) 504 = 19656. From these, we find that the six-digit numbers 523152 and 523656 are multiples of 504. The moral of the story is to keep an open mind as to when to use different methods to solve a problem.",
        "13.23 Let's take a look at the three simplest examples:"
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        "13.24 Let the base-b digits of n be (from right to left) \\(d_0\\), \\(d_1\\), \\(d_2\\), ... Since \\(b \\equiv 1 \\pmod{b - 1}\\), we have",
        "13.25 First, we note that when n = 2, \\(n^3 - n = 6\\). This means that k | 6. With this in mind, we check to see if \\(n^3 - n\\) is always a multiple of 2 and 3. There are several possible ways to go about this, but we take a factoring approach:",
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        "13.27 Let \\(d_0, d_1, d_2, \\ldots\\) be the base-12 digits of \\(n\\) so that",
        "13.28 The last four digits of an integer determine its modulo-\\(2^4\\) residue. This motivates us to examine possible integers modulo 16. This seems reasonable since any power of 2 with at least 4 digits is a multiple of 16.",
        "13.29 Since \\(15 = 3 \\cdot 5\\), and \\(gcd(3, 5) = 1\\), we use the divisibility rules for 3 and 5 to help us find \\(n\\). Since \\(5 \\mid n\\), \\(n\\) has a units digit of either 0 or 5. However, we are told that all of \\(n\\)'s digits are either 0 or 8, so the units digit of \\(n\\) is 0. Now we note that \\(3 \\mid n\\), so the sum of \\(n\\)'s digits is a multiple of 3. However, the sum of \\(n\\)'s digits is also the number of 8's that are digits of \\(n\\). This smallest total of 8's is 3, where \\(3 \\cdot 8 = 24\\) is a multiple of 3. Finally, we want the smallest possible value of \\(n\\), so we include no more digits and \\(n = 8880\\) is the smallest positive multiple of 15 all of whose digits are 0 or 8. The answer to the problem is \\(n/15 = 8880/15 = \\underline{592}\\).",
        "13.30 We could solve all parts of this problem by dividing each of the four integers into longer strings of 1's until no remainder is left:"
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        "1.1 ≡ 1 (mod 11)  ⇒  1⁻¹ ≡ 1 (mod 11)",
        "2.6 ≡ 1 (mod 11)  ⇒  2⁻¹ ≡ 6 (mod 11)",
        "3.4 ≡ 1 (mod 11)  ⇒  3⁻¹ ≡ 4 (mod 11)",
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        "14.21 Modulo-20 residues with no inverses are the ones not relatively prime to 20. Those are 0, 2, 4, 5, 6, 8, 10, 12, 14, 15, 16, and 18.",
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        "14.25 From the first two congruences, we know that",
        "14.26 We are looking for integers \\(n\\) such that \\(200 \\leq n \\leq 500\\) and",
        "14.27 We are looking for the smallest positive integer \\(x\\) that satisfies the system of linear congruences"
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        "14.30 We want to identify mutual days of rest so that we can count them. Days of rest for Adam are in the form 4a for some positive integer a. Days of rest for Ben are in the form 10b - 2, 10b - 1, or 10b where b is a positive integer. We want days when Adam's and Ben's time off overlap. We want solutions, n, that satisfy any one of the three following systems of linear congruences:",
        "14.31 Let A be the number of four-digit integers that leave a remainder of 2 when divided by 7. Let B be the number of four-digit integers that leave a remainder of 4 when divided by 5. Let C be the number of four-digit integers that both leave a remainder of 2 when divided by 7 and a remainder of 4 when divided by 5. Our goal is to compute \\((A - C) + (B - C)\\) (make sure you see why).",
        "14.32 The product of two modulo 120 units digits has a units digit of 1 if the units digits are modulo 120 inverses of each other. Only integers relatively prime to 120 have modulo 120 inverses. We count"
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        "15.20 We need to compute",
        "15.21 We are told that the sum of two primes is equal to a third, so for prime numbers \\(a, b\\), and \\(c\\),",
        "15.22 The smallest such integer is 2457. If our number were 4 or more times another number, then it would be at least 4.2457 = 9828, which is too big. So our number is 2 or 3 times another number. Also note that the smaller number (the number we are multiplying 2 or 3 by) must begin with 2, because 2.4257 = 8514 is too big."
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        "15.23 Since each of the two products includes 1212, we can factor to make things easier:",
        "15.24 All the numbers in this problem are powers of 2, so we can change everything into forms of powers of 2 in order to get the clearest look at the equation:",
        "15.25 We can start applying divisibility rules for each of the smallest primes in order, using our understanding of modular arithmetic to help.",
        "15.26 Since the product \\(8^{12} \\cdot 25^8\\) is a multiple of several powers of 10, it makes sense to reorganize the product in this way:",
        "15.27 We are looking for the smallest positive integer n such that"
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        "15.28 Once again, we apply parity to a sum of primes. Since the sum of the three primes is even, one of them must be the even prime, 2. This means the sum of the other two primes is 22. Our possible choices for the remaining prime numbers are 3 and 19, 5 and 17, and 11 and 11. There are six possible orderings of (2, 3, 19), six possible orderings of (2, 5, 17), and three possible orderings of (2, 11, 11), for a total of 15 ordered triples.",
        "15.29 Solving an equation based on this problem would be tough, but we can make casework relatively easy by narrowing the range of integers whose cubes have four-digits. Since \\(10^3 = 1000\\) and \\(22^3 = 10648\\), there are only twelve 4-digit numbers which are cubes at all. We want the largest four-digit number equal to the cube of the sum of its digits, so we can work downwards from cubes of possible digit sums:",
        "15.30 We see powers of 61 multiplied by 1, -3, 3, and -1. These remind us of the coefficients of \\((x - 1)^3\\):",
        "15.31 Let's call the number we are looking for N. For each m = 2, 3, ..., 10 we have"
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        "15.32 While it's usually easiest to find the GCD of a pair of integers using prime factorization, there is a nice relationship between the integers in this problem: \\(39654 = 39600 + 54 = 50 \\cdot 792 + 54\\). This allows an easy application of the Euclidean algorithm:",
        "15.33 If A and B were both odd, then A + B would be the even prime 2. But 2 is less than the sum of any primes, so either A or B must be even and therefore 2. Since A - B is prime, A > B, so B = 2.",
        "15.34 Let a, b, and c be integers such that \\(0 < a < b < c\\) and \\(a^2 + b^2 + c^2 = 125\\). Since",
        "15.35 We see powers of 3, so we change the integers to exponential form for easier computation:",
        "15.36 We can use the relationship between \\(2^{2006}\\), \\(5^{2006}\\), and the fact that the decimal system is based on powers of 10. We note that \\(2^{2006} \\cdot 5^{2006} = 10^{2006}\\) is a 2007-digit number. We can write the powers of 2 and 5 in a way that makes the last step clear:"
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        "15.37 If \\(n\\) is one less than a multiple of each of 3, 5, 7, 9, and 11, \\(n + 1\\) must be a multiple of all of them, and therefore a multiple of their LCM. Since 3 is a divisor of 9, we can ignore it in the calculation. Since 5, 7, 9, and 11 are all coprime in pairs, the LCM is",
        "15.38 We recall that any number ending in 2 or 4 must be divisible by 2, and any number ending in 5 must be divisible by 5. Since two prime numbers are formed, one of them must end in one of those three numbers. Since a prime can't be divisible by 2 unless it is equal to 2 or by 5 unless it is equal to 5, one of the primes must be either 2 or 5, and the other prime must end in 7. If one of the primes is 2, the other prime can either be 457 or 547. If one of the primes is 5, the other can either be 247 or 427. 247 is not prime, and the smallest of 2 \\(\\cdot\\) 457, 2 \\(\\cdot\\) 547, and 5 \\(\\cdot\\) 427 is 2 \\(\\cdot\\) 457 \\(\\cdot\\) \\(\\underline{914}\\).",
        "15.39 Factoring thus difference of squares, we have",
        "15.40 There are not very many two-digit numbers that are 5 more than a multiple of 7, so we can just list them out:",
        "15.41 We can use what we know about the value of the square to narrow down the possibilities. We start with broad estimates: \\(300^2 = 90,000 < 152AB1 < 160,000 = 400^2\\), so the first digit of the square root is 3. Indeed, \\(380^2 = 144,400 < 152AB1\\), so the first two digits of the square root are either 38 or 39. Since the last digit of the number 152AB1 is 1, its square root must have a last digit of either 1 or 9. Since 152AB1 is close to midway between 144,400 and 160,000, the two possibilities we must check for its square root are 389 and 391. \\(389^2 = 151,321 < 152AB1\\), but \\(391^2 = 152881\\), which is of the required form, so the answer is \\(3 + 9 + 1 = \\underline{13}\\).",
        "15.42 Seeing all those powers of 5, we rewrite the whole expression in terms of 5:"
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        "15.43 When an integer n is 1 more than a multiple of each 2 and 3, then n - 1 is a multiple of both 2 and 3 and therefore a multiple of 6. We write \\(n = 6m + 1\\) for a nonnegative integer m. Since \\(0 < n < 100\\),",
        "15.44 Since the sum of the two prime numbers is 20, they can be expressed as \\(10 + x\\) and 10 - x for some integer x. We can compute the product as a difference of squares:",
        "15.45 Notice that \\((10)(6) = 2 \\cdot (5)(6)\\). The squares of two numbers and two times their product remind us of the factorization \\((x - y)^2 = x^2 - 2xy + y^2\\). When x = 5 and y = 6, we get the expression we are trying to evaluate:",
        "15.46 We recall that a number is a multiple of 5 if and only if its units digit is 0 or 5. Therefore, none of the given 2, 4, or 8 can be the last digit. We order 2, 4, and 8 from least to greatest to minimize the integer, so the smallest such number will be of the form 248x. The smallest possibility is \\(\\boxed{2480}\\).",
        "15.47 We factor \\(66666 = 3 \\cdot 22222\\). Then \\(A = 66666^4 = (3 \\cdot 22222)^4 = 3^4 \\cdot 22222^4 = 81B\\). Dividing by B we get \\(A/B = \\boxed{81}\\).",
        "15.48 Since \\(2^4 = 16\\), we can use a difference of squares twice:",
        "15.49 The temptation is to write out all of the first 25 primes, look at all of their remainders when divided by 4, and multiply, but there is a much simpler way. Since 2 is a prime, the product of the first 25 primes is going to be a multiple of 2, so its remainder when divided by 4 will be either 2 or 0. But this product is not a multiple of 4, since no other primes have a factor of 2, so the remainder cannot be 0. Therefore, the remainder is \\(\\boxed{2}\\)."
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        "15.50 Rather than exponentiating and subtracting, we factor \\(2^{19}\\) out of each term, getting",
        "15.51 Let N > 1 be an integer that, when divided by each of 2, 3, ..., 10 and 11, leaves a remainder of 1.",
        "15.52 The repeated digits give us a clue about a common factor. We write \\(8008=8\\cdot1001\\), and \\(14014=14\\cdot1001\\). Since, gcd(8, 14) = 2,",
        "15.53 Any multiple of \\(9^{3}\\) can be written in the form \\(9^{3}n\\) for some integer n. Our goal is to count the number of integers in this form between \\(9^{4}\\) and \\(9^{6}\\). We divide the inequality",
        "15.54 Those coefficients look familiar (a row of Pascal's triangle!), and the fact that the powers of 53 decrease while those of 3 increase gives it away: the given expression is the expansion of \\((53-3)^{5}\\). Now the computation is relatively simple:",
        "15.55 We can use both factorization and the trick for squaring integers with units digits of 5."
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        "15.56 Since each denominator is at least 1, we can determine the value of each variable in succession by subtracting out the largest positive integer less than or equal to the right-hand side of the equation.",
        "15.57 There are a couple of nice approaches to this problem. We could start by factoring the difference of cubes, but since the numbers being used are constructed from such easy-to-multiply integers, we can work the problem algebraically.",
        "15.58 We can cancel \\(2^{2000}\\) from every part of the fraction for easy computation:",
        "15.59 Seeing the powers of 18 makes us think about a factorization of the cube of a sum. However, the “coefficients” don’t match. We notice powers of 2 added – particularly the last term, \\(2^3\\). Indeed, we can compute this as the cube of a sum:",
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        "15.61 The fraction \\(\\frac{n - 13}{5n + 6}\\) is reducible if and only if \\(\\gcd(5n + 6, n - 13) > 1\\). Using the Euclidean algorithm,",
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