For universal set U, and sets A,B which are subsets of U, the following information is given
n(U)=47,
n(A)=18,
n(B)=11,n(A∩B)=10
Then, the number of elements that are neither in A nor B is
n(Ac ∩ Bc) = n(A U B)c [De Morgan's laws]
= n(U) - n(A U B)
=n(U)−[n(A)+n(B)−n(A∩B)]
=47−[18+11−10]
=47−19
=28