posted by David .
Two independent events, A and B, satisfy Pr(A) = 0.6 and Pr(B) = 0.3. In the case of Pr(A or B), why isn't the answer 0.3? We are looking at the probability of A or B. I choose B.
The answer is Prob(A) + Prob(B) - Prob (A + B)= 0.6 + 0.3 - 0.18 = 0.72
The last term avoids double counting of the occurence of both A and B, which satisfied the "A or B" criterion only once.
The answer is certainly not 0.3, since 60% of the events are A and satisfy the A or B requirement