300
From de-Morgan's law of complementation, we have A′∩B′=(A∪B)′.
⇒n(A′∩B′)=n((A∪B)′)
But, n((A∪B)′)=n(U)−n(A∪B) by definition of 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