1. The problem statement, all variables and given/known data Suppose that p and q are prime numbers and that n = pq. Use the principle of inclusion-exclusion to find the number of positive integers not exceeding n that are relatively prime to n. 2. Relevant equations Inclusion-Exclusion 3. The attempt at a solution The way I see it, n = pq is contained in a set of all the prime numbers from 1 to pq, plus the multiples that are not prime. So: n = pq - |pi U qi| I'm not exactly sure where to go from here, though. Any help is appreciated.