MathDB
Very hard(at least for me)

Source: 4th JBMO 2000, Problem 4

June 10, 2004
inequalitiescombinatorics solvedcombinatorics

Problem Statement

At a tennis tournament there were 2n2n boys and nn girls participating. Every player played every other player. The boys won 75\frac 75 times as many matches as the girls. It is knowns that there were no draws. Find nn. Serbia