Answer
$52$
Work Step by Step
$A$ or $B$ or $C$, which is $A\cup B\cup C$, is the set that contains all the elements that are either in A or in B or in C.
The number of elements in this set can be found by adding all the numbers, while using each number in a region only once, inside the circles that represent the sets $A$, $B$, and $C$.
Thus, using the diagram: $n(A\cup B\cup C)=15+3+10+2+5+2+15=52$
