Answer
18
Work Step by Step
In order to count all of the elements in set A but not in set C, we have to add up the numbers that are in A, not including those which are in C. This means:
$= 15 + n(A\cap B\cap C)+n(A\cap B)$ + $n(A\cap C) - n(A\cap B\cap C) - n(A\cap C)=15+n(A\cap B)=15+3=18$
