1998 AIME Problem 4

Attempt Problem 4 of the 1998 AIME 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 1998 AIME solutions, or check the answer key.

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4.

Nine tiles are numbered 1,2,3,,9,1, 2, 3, \ldots, 9, respectively. Each of three players randomly selects and keeps three of the tiles, and sums those three values. The probability that all three players obtain an odd sum is mn,\frac{m}{n}, where mm and nn are relatively prime positive integers. Find m+n.m + n.

Answer: 17
Concepts:basic probabilityparitycombinations
Difficulty rating: 2350
Solution:

A player's three tiles have an odd sum exactly when the player holds an odd number of odd tiles — one or three. The nine tiles include five odd and four even, and the only way to split five odd tiles into three groups of size one or three is 3+1+1.3 + 1 + 1.

Count favorable deals: choose which player gets three odd tiles (33 ways), choose that player's odd tiles ((53)=10\binom{5}{3} = 10 ways), give one of the two remaining odd tiles to each other player (22 ways), then split the four even tiles two and two between those players ((42)=6\binom{4}{2} = 6 ways), for 31026=3603 \cdot 10 \cdot 2 \cdot 6 = 360 deals. The total number of deals is (93)(63)=8420=1680.\binom{9}{3}\binom{6}{3} = 84 \cdot 20 = 1680.

The probability is 3601680=314,\frac{360}{1680} = \frac{3}{14}, so m+n=3+14=17.m + n = 3 + 14 = 17.

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