11 students in a test, for any 2 qestions exactly 6 solved 1 correctly
Source: KJMO 2005 p4
May 1, 2019
combinatorics
Problem Statement
students take a test. For any two question in a test, there are at least students who solved exactly one of those two questions. Prove that there are no more than questions in this test. Showing the equality case is not needed.