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08-10-2010, 04:16 PM
I can't seem to remember how to determine what P(A|B) is if the two events are determined to be mutually exclusive with probabilities P(A)= 0.3 and P(B)= 0.2 The question before asked what P(A and B) was and since they are mutually exclusive I put 0. I'm not sure if I should add 0.3 and 0.2 and divide by 0.2 (P(A and B) divided by P(B)) or if i should put 0 divided by 0.2. I can't find the formula for how this works. Any help would be really appreciated!! Thanks ahead of time!!

Dason
08-10-2010, 04:30 PM
The question before asked what P(A and B) was and since they are mutually exclusive I put 0. I'm not sure if I should add 0.3 and 0.2 and divide by 0.2 (P(A and B) divided by P(B))
Are you trying to say you think P(A and B) = .3 + .2 (hint: this isn't correct)

or if i should put 0 divided by 0.2. I can't find the formula for how this works. Any help would be really appreciated!! Thanks ahead of time!!

This is one way to do it. Because P(A and B) = 0 and we know that P(A|B) = P(A and B)/P(B) so what you wrote is correct. Basically it comes down to the fact that if we're thinking about the conditional probability P(A|B) and we know A and B are mutually exclusive then P(A|B) is asking "whats the probability of A if we know B happened" but we know that they can never occur at the same time so it's 0.