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2022 AMC 10B Problem 8

Problem 8 of 25EasierNumber TheoryProblem-Solving Techniques

Consider the following 100100 sets of 1010 elements each: {1,2,3,…,10},{11,12,13,…,20},{21,22,23,…,30},⋮{991,992,993,…,1000}.\begin{gathered} \{1,2,3,\ldots,10\},\\ \{11,12,13,\ldots,20\},\\ \{21,22,23,\ldots,30\},\\ \vdots\\ \{991,992,993,\ldots,1000\}. \end{gathered} How many of these sets contain exactly two multiples of 7?7?

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Solution

A block of ten consecutive integers contains exactly two multiples of 77 precisely when its first multiple of 77 is in one of the first three positions. Thus that multiple must have units digit 1,1, 2,2, or 3.3. The multiples of 77 with those units digits are, respectively, 21+70j,42+70j,63+70j.\begin{gathered}21+70j,\\42+70j,\\63+70j.\end{gathered} For each expression, j=0,1,…,13j=0,1,\ldots,13 gives a value at most 1000,1000, so each class contributes 1414 blocks. Therefore, the total is 3⋅14=42.3\cdot14=42. Thus, the answer is B .
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