Question

If X and Y are two sets such that X ∪Y has 18 elements, X has 8 elements and Y has 15 elements; how many elements does X ∩Y have?

Answer 1

It is given that:

n(X ∪Y) = 18, n(X)=8, n(Y) = 15

n(X ïf ? Y) = ?

We know that:

n(X ∪Y)=n(X)+n(Y)-n(X∩Y)

18=8+15-n(X∩Y)

n(X∩Y)=5