
### Instruction: help me answer this 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?, think step by step
### Assistant: Using the formula n(X ∪ Y) = n(X) + n(Y) – n(X ∩ Y) , we find n(X ∩ Y) = 5 , The answer is n(X ∩ Y) = 5
