a mathematics competition had 9 easy and 6 difficult problems
Source: New Zealand NZMOC Camp Selection Problems 2015 p2
September 19, 2021
combinatorics
Problem Statement
A mathematics competition had easy and difficult problems. Each of the participants in the competition solved of the problems. For each pair, consisting of an easy and a difficult problem, the number of participants who solved both those problems was recorded. The sum of these recorded numbers was . How many participants were there?