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

Explanation

To find the number of elements in the intersection of sets X and Y, we can use the formula for the union of two sets:

\[ n(X \cup Y) = n(X) + n(Y) - n(X \cap Y) \]

Where:
- \( n(X \cup Y) \) is the number of elements in the union of sets X and Y.
...