several people visited a library yesterday
Source: 1950 Hungary - Kürschák Competition p1
October 10, 2022
combinatorics
Problem Statement
Several people visited a library yesterday. Each one visited the library just once (in the course of yesterday). Amongst any three of them, there were two who met in the library. Prove that there were two moments and ' yesterday such that everyone who visited the library yesterday was in the library at or (or both).