2023 AIME I Problem 1
Attempt Problem 1 of the 2023 AIME I below, then check your answer against the professionally curated solution from LIVE by Po-Shen Loh. You can also try the full timed exam, view all 2023 AIME I solutions, or check the answer key.
All problems are used with official legal permission of the Mathematical Association of America (MAA).
1.
Five men and nine women stand equally spaced around a circle in random order. The probability that every man stands diametrically opposite a woman is where and are relatively prime positive integers. Find
Answer: 191
Small hint:
The positions split into diametrically opposite pairs, and only the set of positions the men occupy matters.
Big hint:
Count sets where no pair has two men: choose of the pairs to hold a man, then a position in each; compare with
Solution:
The positions split into diametrically opposite pairs. Only the set of positions occupied by the men matters, and all five-element sets are equally likely. Every man stands opposite a woman exactly when no pair contains two men, so choose which of the pairs contain a man ( ways) and which position of each chosen pair the man occupies ( ways), for favorable sets.
The probability is so
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