China Mathematical Olympiad 1993 problem5
Source: China Mathematical Olympiad 1993 problem5
September 23, 2013
combinatorics unsolvedcombinatoricseasy
Problem Statement
students bought some books in a bookstore. It is known that every student bought exactly three kinds of books, and any two of them shared at least one kind of book. Determine, with proof, how many students bought the most popular book at least? (Note: the most popular book means most students bought this kind of book)