Perrybhadra is right, but if it helps you understand better, here is another way: P(not A and not B) =1- P (A union B) = 1 - [ P(A) + P(B) - P(A and B) = 1- P(A) -A(B) + P(A)P(B) (if A and B are independent) *you should get the same answer both ways.