Try the binomial probability formula:
P(x) = (nCx)(p^x)[q^(n-x)]
n = 15
x = 4
p = .20
q = 1 - p = .80
Substitute the values into the formula and calculate your probability.
there is a 1/5 (.20) probability that one type of appliance will break with in two years. find the probability that 4 out 15 such appliances willl break within that time period
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