# Math Help - [SOLVED] bayes' rule

1. ## [SOLVED] bayes' rule

If I have e.g. P(A|B) is 0.9 and P(not A|not B) = 0.9 then how do I find P(A|not b) and P(not A|B)? Also, how do I find the P(A)?

2. Originally Posted by joop
If I have e.g. P(A|B) is 0.9 and P(not A|not B) = 0.9 then how do I find P(A|not b) and P(not A|B)? Also, how do I find the P(A)?
Draw a tree diagram. The first two branches are B and B'. Then from each of these branches you have the two branches A and A'.

It should then be clear that Pr(A | B') = 0.1 and Pr(A' | B) = 0.1.

There's not enough information to calculate a numerical value of Pr(A).

3. I solved for P(A) by drawing a diagram (P(B) was given to us). Thanks a lot though.