Prob something occurs given something else occurs

[Maybe_Memorie]

So suppose we have 3 boxes, A,B,C. Black and Red balls in each.
In A, 3R and 7B.
B, 6R and 4B
C, 8R and 2B

First you take a ball from A. If its R, take one from B. If its B, take one from C.

Given the second is R, what is the prob the first is R?

So I have a tree diagram, but I'm quite unsure about what to do


Also, let's say we have the length of time of telephone calls associated with 2 products.
Mean and standard deviation given.
If you want to find the prob that a random call will take less than 7 minutes, are you supposed to add the mean and SDs together? then proceed as usual?

 

[monomath]

Here's a link to the wikipedia page on conditional probabilityhttp://en.wikipedia.org/wiki/Conditional_probability#Definition

Have you identified (the probabilities of) the different events and their intersection?