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2022 AMC 12A Problem 10

Problem 10 of 25EasierCombinatorics

What is the number of ways the numbers from 11 to 1414 can be split into 77 pairs such that for each pair, the greater number is at least 22 times the smaller number?

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Solution

Any number 88 or larger cannot be a smaller element (its double exceeds 1414), so 88–1414 are all larger elements and 11–77 are all smaller elements. Match each smaller ss to a larger g≥2s.g\ge2s. Processing from the most restrictive: s=7s=7 forces g=14g=14 (11 way); then s=6s=6 has {12,13}\{12,13\} left (22); s=5s=5 has 3;3; s=4s=4 has 4;4; s=3s=3 has 3;3; s=2s=2 has 2;2; s=1s=1 has 1.1. The number of matchings is 1⋅2⋅3⋅4⋅3⋅2⋅1=144.1\cdot2\cdot3\cdot4\cdot3\cdot2\cdot1=144. Thus, the correct answer is E.
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Tagged: arrangements with restrictions · multiplication principle

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