Question 1047211
AUB is the set of elements of S that are part of A or B, or both.
A∪B = {1,2,3,5,7,9,10,11,12,15,16,17,18}
I could count them, but there are too many, and I have not had my first cup of coffee yet.
A∪B is all of S except 4, 6, 8, 13, 14, 19, and 20.
S had 20 elements, and 7 were left out, so
n(A∪B) = 20-7 = {{{highlight(13)}}} .
A∩B is the set of elements of S that are common to A and B.
A∩B = {2,3,7} , so  n(A∩B) = {{{highlight(3)}}} .