n Q ∩ R there are 9 elements.

In Q ∩ S there are 4 elements.

In R ∩ S there are 11 elements.

If (Q ∩ R) ∪ (Q ∩ S) ∪ (R ∩ S) includes 18 elements, how many elements are in Q ∩ R ∩ S?