A school class has 90 students, each of whom has ten friends among the other students. Prove that each student can invite three people at a restaurant so that each of the four people at the table will know at least two of the other three. If this had to be translated to a graph it would have 90 vertices and each vertex would have 10 edges to represent friendships. I assume that when a person invites 3 other people he has an edge connecting him to the other three, so this means within such a group of 4 everybody knows at least 1. I guess the solution will have to involve the Pigeonhole principle in some form.