Yaphp!!!
Source: RMO 1990 Problem 1
October 14, 2005
pigeonhole principle
Problem Statement
Two boxes contain between them 65 balls of several different sizes. Each ball is white, black, red or yellow. If you take any five balls of the same colour, at least two of them will always be of the same size(radius). Prove that there are at least three ball which lie in the same box have the same colour and have the same size(radius).