STEP 1: Drawing tree This is a probability question so it is always helpful to draw a tree. Refer to slide number 3. The 1st branch consists of 2 cases which are he’s the father (F) or maybe he’s not (NF). The next cases branching out from those are whether the test was positive (P) or negative (N) in determining the last case.
STEP 2: Find the probabilities of the different cases -refer back to tree -find P(FP), we multiply the two values because we want the probability of him being the father (.2) and the test being positive (.99) -keyword is and meaning to multiply -same process for rest of cases
STEP 3: Find cases on branch where he tested positive: -question states the paternity test was positive -circle the branches where the outcome was a positive -the case of him testing positive is true in 2 branches, which are P(NFP) and P(FP) -whenever a case fits on more than one branch, it is a given that question -since you want to find whether he tested positive and is the father you are looking for P(F/P) -now all you need to do is plug in the values of P(FP) and P(NFP) -refer to slide 4 for the work