Let n(M ∪ N ) = x
Then 20 - x + x + 30
- x = n(M ∪ N)
50 - x = 40
50 - 40 = x
10 = x
x = 10
Hence, n(M U N)' = 8 + (20 - 10) + (30 + 10)
= 8 + 10 + 20
= 38