



 
Hi Danielle. How many ways can the test give her the result that H.I.V. is present?
4% of the people who take the test actually have H.I.V. Among those, the test detects it 95% of the time. So the chances that (a) happens is 4% x 95%. There is a 96% chance that she doesn't have H.I.V., but when uninfected people are tested, 1.25% of them get false positive results. So the chances that (b) happens is 96% x 1.25%. The question asks what is the probability that (a) happens rather than (b). Can you finish the question now?  


Math Central is supported by the University of Regina and The Pacific Institute for the Mathematical Sciences. 