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Fair game - ISL 1986

Source:

August 31, 2010
probabilitycombinatoricsgamegame strategycountingIMO Shortlist

Problem Statement

Three persons A,B,CA,B,C, are playing the following game:
A kk-element subset of the set {1,...,1986}\{1, . . . , 1986\} is randomly chosen, with an equal probability of each choice, where kk is a fixed positive integer less than or equal to 19861986. The winner is A,BA,B or CC, respectively, if the sum of the chosen numbers leaves a remainder of 0,10, 1, or 22 when divided by 33.
For what values of kk is this game a fair one? (A game is fair if the three outcomes are equally probable.)