What you really want to find is .
You know that .
So
Here is the problem I have...
A certain virus infects one in every 400 people. A test used to detect the virus in a person is positive 92% of the time if the person has the virus and 4% of the time if the person does not have the virus. (This 4% is called a false positive.) Let A be the event "the person is infected" and B be the event "the person tests positive." Find the probability, given a person tests positive, that the person really is infected.
For some reason I cannot get the multiplication rule while everything else in stats has been a breeze. If someone could just tell me which numbers I use I'm sure I'll be good to go for awhile.