Problem

Source: 2021 Israel National Olympiad P6

Tags: combinatorics, Tournament graphs



21 players participated in a tennis tournament, in which each pair of players played exactly once and each game had a winner (no ties are allowed). The organizers of the tournament found out that each player won at least 9 games, and lost at least 9. In addition, they discovered cases of three players $A,B,C$ in which $A$ won against $B$, $B$ won against $C$ and $C$ won against $A$, and called such triples "problematic". a) What is the maximum possible number of problematic triples? b) What is the minimum possible number of problematic triples?