a) P(positive test result | subject uses drugs) = P(positive
test result and subject uses drug) / P(subject uses drug)
                                                               Â
= (8/200) / ((8+2)/200)
                                                               Â
= 8/10
                                                               Â
= 0.8
P(subject uses drug | positive test result) = P(subject uses
drug and positive test result) / P(positive test result)
                                                        Â
= (8/200) / ((8+10)/200)
                                                        Â
= 8/18
                                                        Â
= 4/9 or 0.444