Putnam US PigeonholePrinciple TheColoringMethod
Basic
1953

Problem - 2748
There are $6$ points in the $3$-D space. No three points are on the same line and no four points are one the same plane. Hence totally $15$ segments can be created among these points. Show that if each of these $15$ segments is colored either black or white, there must exist a triangle whose sides are of same color.
The solution for this problem is available for $0.99. You can also purchase a pass for all available solutions for$99.