Mathematics for IGCSE & O level - Sets (Section 3 - No. 2)

If n(A) = 5 and n(B) = 7, what is the maximum possible value of n(A ∪ B)?

The maximum occurs when the sets are disjoint, so n(A ∪ B) = n(A) + n(B) = 5 + 7 = 12.