Prob (first defective) = 4 / 20 = 1/5
Prob(2nd defective) = 3/19
Prob (3rd defective) = 2/18 = 1/9
Pron 4th defective) = 1/ 17
Prob ( all 4 defective) = 1/5 * 3/19 * 1/9 * 1/17 = 1 / 4845 answer
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