If A and B are 2 sets such that A ∪ B has 40 elements, A has 18 elements and B has 29 elements, how many elements does A ∩ B have?
A set S has 7 elements. How many subsets having at most 5 elements does it have?
If A and B are 2 sets such that A U B has 40 elements, A has 18 elements and B has 29 elements, how many elements does A ∩ B have? __