IMO 1979 LL BUL13
The plane is divided into equal squares by parallel lines; i.e.,
IMO 1979 LL BUL13
Origin: BUL
Problem
The plane is divided into equal squares by parallel lines; i.e., a square net is given. Let M be an arbitrary set of n squares of this net. Prove that it is possible to choose no fewer than n/4 squares of M in such a way that no two of them have a common point.