blissdismissed

I was doing a GREmath practice book and came across this question. P(A or B) = 0.60 and P(A) = 0.20 a. Find P(B) given that events A and B are mutually exclusive. b. Find P(B) given that events A and B are independent. For part a. I understand, P(A or B) = P(A) + P(B) therefore, P(B) is 0.40, and that agrees with the answer key. But, for part b. I'm not sure whats going on. I feel like it must be something simple that I'm missing. For independent cases I know that P(A and B) = P(A)P(B), but I don't see how to get P(B) The answer key says that for part b, P(B) is 0.50, could anyone help me in understandin this one? Thanks