Mathematics for IGCSE & O level - Sets (Section 3 - No. 50)

If n(U) = 20, n(A) = 8, and n(B) = 10, and n(A ∩ B) = 3, what is n((A ∪ B)')?

n(A ∪ B) = n(A) + n(B) - n(A ∩ B) = 8 + 10 - 3 = 15. Then n((A ∪ B)') = n(U) - n(A ∪ B) = 20 - 15 = 5.