If n(U)=700, n(A)=200, n(B)=300 and n(A∩B)=100, then n(A′∩B′)= ___.
300
By de-Morgan's law of complementation,
n(A′∩B′)=n((A∪B)′)
But, by the definition of a complement of a set,
n((A∪B)′)=n(U)−n(A∪B).
=n(U)−[n(A)+n(B)−n(A∩B)]
=700−[200+300−100]
=300.