Problem

Source: 2017 Iran MO 3rd round, second combinatorics P3

Tags: Iran, combinatorics



$30$ volleyball teams have participated in a league. Any two teams have played a match with each other exactly once. At the end of the league, a match is called unusual if at the end of the league, the winner of the match have a smaller amount of wins than the loser of the match. A team is called astonishing if all its matches are unusual matches. Find the maximum number of astonishing teams.