Question

If n(U)=40,n(AUB)=31.Find n(A'nB')

Answer 1

n(A'nB')=n(AUB)' ----->De Morgan's law. 
Therefore, n(AUB)'=n(U)-n(AUB)=40-31=9 
So, n(A'nB')=n(AUB)'=9

Answer 2

The answer is: (A'nB')=9