If n(U) = 50, n(A) = 25, and n(B) = 20, where U is the universal set. If A and B are disjoint, find n(A ∪ B)'.
Answer: A
If sets A and B are disjoint, their intersection is empty, and n(A ∩ B) = 0.
The union of disjoint sets is n(A ∪ B) = n(A) + n(B) = 25 + 20 = 45.
The complement of the union, n((A ∪ B)'), is the number of elements in the universal set but not in the union.
n((A ∪ B)') = n(U) - n(A ∪ B) = 50 - 45 = 5.