Olympiad Maths Prep

Track / Stage 5 / 230 of 400 #830 of 2000

Problem 830

AIME late
Combinatorics Difficulty 5.5 Find the answer

【Example 6】Two first-year high school students participate in a chess competition for second-year high school students. During the competition, every two players play one game, the winner gets 1 point, the loser gets 0 points, and if it's a draw, each gets 1/21 / 2 point. It is known that the two first-year students scored a total of 8 points, and each second-year player scored the same and an integer number of points. How many second-year students participated in the competition?

Official solution

Let the number of participants from the second year be nn, so together with the first year, there are n+2n+2 participants, hence the number of matches played is Cn+22C_{n+2}^{2}, and the total score of all participants is Cn+22C_{n+2}^{2}. Among these, 8 points were scored by the first-year students, and the remaining Cn+228C_{n+2}^{2}-8 points were evenly distributed among the second-year students. Therefore, according to the problem, we have
Cn+228n= a non-negative integer  \frac{C_{n+2}^{2}-8}{n}=\text { a non-negative integer }

which means
n2+327n= a non-negative integer  \frac{n}{2}+\frac{3}{2}-\frac{7}{n}=\text { a non-negative integer }

Thus, if nn is odd, then 7/n7 / n must be an integer, which implies n=7n=7 or 1. However, n=1n=1 contradicts the fact that the two first-year students scored a total of 8 points, so n=7n=7; if nn is even, then 3/27/n3 / 2-7 / n must be an integer, which implies 7/n7 / n must be a reduced fraction with a denominator of 2, hence n=14n=14.

Source: NuminaMath-1.5, licensed Apache-2.0. Statement and solution reproduced as published; topic, difficulty and ordering added by this site.