College Algebra (10th Edition)

In order to count all the elements in sets A and B, (meaning A union B) we have to add up the numbers in the circle of both A and B. This means: $n(A \cup B)=10+15+n(A\cap C)+n(A\cap B\cap C)+n(B\cap A)+n(B\cap C)$ $n(A\cup B)=10+15+2+5+3+2=37$