If I understand the notation being used, B' is the complement of B. So you are correct in that B' is all of the positive integers not divisible by 5 up to and including 100.

However, the solution is using a shorthand approach to solve n ( A intersect B' ) by understanding that this is asking for the number of elements in A that are not in B.

Since you already determined that there are 6 elements in both A and B, then the number of elements that are just in A is 33 - 6.

However, the solution is using a shorthand approach to solve n ( A intersect B' ) by understanding that this is asking for the number of elements in A that are not in B.

Since you already determined that there are 6 elements in both A and B, then the number of elements that are just in A is 33 - 6.