[mosdef]

Let's try this a different way.

Prob ( A or B ) = 1 - Prob ( A' and B') = 0.8

Prob ( A or B ) = Prob (A) + Prob (B) - Prob (A and B)

Prob (A and B) = 0.5 + 0.35 - 0.8 = 0.05

Basically, for A or B to happen, add up the probabilities of A and B, but the subtract the "A and B" case so that you don't double count.