Question 1100861
<br>It's rather hard to interpret the information as you present it.  Perhaps some formatting got lost in cyberspace....?<br>
Apparently we have a shipment of computers....
15 with 80GB hard drive and 2GB memory
55 with 120GB hard drive and 2GB memory
10 with 80GB hard drive and 4GB memory
20 with 120GB hard drive and 4GB memory<br>
The probability P(C|B) is the probability that C is true (the computer has 2GB memory), GIVEN THAT B is true (it has a 120GB hard drive).<br>
With this conditional probability, the sample space is only those computers that have a 120GB hard drive; that is 55+20 = 75 computers.<br>
The desired condition is that it is a computer with a 120GB hard drive AND a 2 GB memory; there are 55 of those computers.<br>
So P(C|B) is 55/75 = 11/15.<br>
Formally, the probability P(C|B) is {{{P(C and B)/P(B)}}}<br>
I personally find this hard to work with.  I find it much easier to think in terms of the sample space consisting of only those computers that satisfy condition B.