MathDB
How many people are at the meeting?

Source: IMO Shortlist 1995, N5

August 10, 2008
combinatoricsgraph theoryExtremal combinatoricsIMO Shortlist

Problem Statement

At a meeting of 12k 12k people, each person exchanges greetings with exactly 3k\plus{}6 others. For any two people, the number who exchange greetings with both is the same. How many people are at the meeting?