A batch of 20 used automobile alternators contains 4 defectives. If 3 alternators are selected at random, find the probability of the events (using the Rule of Combinations); A= None of the defective appears Answer is .491 but I can not figure out how they come up with that! Solution P(None defective) = 16C3/20C3 (As there are 16 non defective alternators and 20 total alternators => Probability = ways of selecting 3 alternatorsout of 16 / ways of selecting 3 alternators out of 20) = 0.491.