1. The problem statement, all variables and given/known data Set A has twice the number of elements as Set B, 1/3 of the elements of Set A are the same as in Set B, the union of A and B is 42, what is the intersection? 3. The attempt at a solution This was one of my exam questions, and I just want to see what the correct answer was. What I tried to do was use the inclusion exclusion principle so |AUB| = |A| + |B| - (1/3)*|A| 42 = |A| + 1/2|A| - (1/3)|A| 42 = (1/6)|A| + (3/6)*|A| - (2/6)*|A| 42 = (1/3)|A| And 42 is the intersection, but that makes absolutely no sense, can anyone show me the correct way to get the answer?