Let n(U) = 700, n(A) = 200, n(B) = 300 and n(A∩B)=100,
Then n(Ac∩Bc)=
400
600
300
200
n(Ac∩Bc)=n(∪)−n(A∪B)
=n(∪)−[n(A)+n(B)−n(A∩B)]
=700−[200+300−100]=300.