sets problem

Question

if A and B are two sets containing 3 and 6 elements, what can be minimum and maximum number of elements in AUB(A Union B)?

Answer 1

If B is a subset of A, then all three of its elements are already contained in A.  Thus AUB would contain only 6 elements (the minimum).  If set B is disjoint from set A, then none of its elements is contained in set A, and AUB would have 9 elements (the maximum).