Terminology alert: We write finite sets as lists of their members (also called elements). For example, {2,3,5,7} is an excellent set. So is {1,4}. These sets are disjoint because they have no members in common. On the other hand, {1,2} is not disjoint from either {2,3,5,7} or {1,4}. The union of two sets A,B (or many sets A,B,...,N) is a set containing all members of A and of B (and of C,...,N). The union of the three sets listed so far is {1,2,3,4,5,7}.
(a) How many elements are in the union of two disjoint finite sets?
(b) How many members does a union of finitely many disjoint finite sets have?
(c) Are the previous two questions related to any of the previous problems?
(d) How many members does the union of n disjoint sets, each with m elements, have?