be the positive integers between 100 and 999 inclusive. contains 900 integers, while we are interested in integers divisible by 7. Thus 128 integers are divisible by 7.
Let be the set of positive integers between 100 and 999. The set contains 900 integers, and the integers are divisible by 7. By division rule, where is the finite set with union of pairwise disjoint subsets with elements.
