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 mn,\frac{m}{n}, where mm and nn are relatively prime positive integers. Find m+n.m + n.
Answer: 191
Concepts:basic probabilitycombinationspairing and grouping
Small hint:
The 1414 positions split into 77 diametrically opposite pairs, and only the set of positions the men occupy matters.
Big hint:
Count sets where no pair has two men: choose 55 of the 77 pairs to hold a man, then a position in each; compare with (145).\binom{14}{5}.
Solution:
The 1414 positions split into 77 diametrically opposite pairs. Only the set of positions occupied by the men matters, and all (145)=2002\binom{14}{5} = 2002 five-element sets are equally likely. Every man stands opposite a woman exactly when no pair contains two men, so choose which 55 of the 77 pairs contain a man ((75)=21\binom{7}{5} = 21 ways) and which position of each chosen pair the man occupies (25=322^5 = 32 ways), for 21⋅32=67221 \cdot 32 = 672 favorable sets.
The probability is 6722002=48143,\frac{672}{2002} = \frac{48}{143}, so m+n=48+143=191.m + n = 48 + 143 = 191.
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