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Syamantak
probability of raining on a day is 0.2 What is the probability of raining on exactly 2 days in a week ?
to chose 2 days in 7 days, use the 7C2 = 7!/(7-2)2! = 7!/5!2! = 21 ways so, the probability of raining on exactly 2 days in a week is 21 * 0.2 * 0.2 = ...
Sorry but it is a wrong answer @RadEn