I have just started studying sets and am not sure how to go about this problem.

It says "find an equality that holds between the quantities #A, #B, #(AUB), #(A-intersection-B)"

I am not sure if this is correct because the answer I have come up with seems so simple, but if A and B were the same sets (or A a subset of B, both of them having exactly the same elements), wouldn't #(AUB) (the number of elements in AUB) be the same as #A, #B and #(A intersection B)???

Thanks a lot!