Question: how many integers from 1 to 50 are multiples of 2 or 3 but not both?
Answer: The Inclusion-Exclusion principle. How many integers from 1 to 50 are multiples of 2 or 3 but not both? From 1 to 100, there are 50/2=25 numbers which are multiples of 2. There are 50/3=16 numbers which are multiples of 3.