Answer
52
Work Step by Step
In order to count all the elements in set that are either in A, B, or C, we have to add up the numbers that are in the union of the three sets. This means: $ n(A\cup B\cup C)=15+10+15+ n(A\cap B) +n(A\cap C) + n(B\cap C) + n(A\cap B\cap C)=52$
