Let S be the universal set, where:
S = {1, 2, 3, ..., 18, 19, 20}
Let set A and B be subsets of S, where:
Set A = {1, 2, 3, 4, 7, 8, 9, 11, 12, 13, 16, 18, 19}
Set B = {2, 3, 4, 6, 9, 11, 12, 13, 18}
Find the following:
The number of elements in set A ∩ B.