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Players in a tournament

Source: Saudi Arabia IMO TST Day IV Problem 2

July 22, 2014
AMCAIMEcombinatorics unsolvedcombinatorics

Problem Statement

In a tournament each player played exactly one game against each of the other players. In each game the winner was awarded 11 point, the loser got 00 points, and each of the two players earned 12\tfrac{1}{2} point if the game was a tie. After the completion of the tournament, it was found that exactly half of the points earned by each player were earned in games against the ten players with the least number of points. (In particular, each of the ten lowest scoring players earned half of his points against the other nine of the ten). What was the total number of players in the tournament?