Mathematics for IGCSE & O level - Sets (Section 3 - No. 7)

If n(A) = 7, n(B) = 9 and n(A ∩ B) = 4, what is n(A ∪ B)?

Use the inclusion-exclusion principle: n(A ∪ B) = n(A) + n(B) - n(A ∩ B) = 7 + 9 - 4 = 12.