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2022 AMC 12A Problem 10

Problem 10 of 25EasierCounting & Probability

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 881414 are all larger elements and 1177 are all smaller elements. Match each smaller ss to a larger g2s.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 1234321=144.1\cdot2\cdot3\cdot4\cdot3\cdot2\cdot1=144. Thus, the correct answer is E.

More practice

Concepts: arrangements with restrictions · multiplication principle

Problem text and solution from the LIVE past-contest archive. See also the AoPS wiki page for community solutions.