Assume that the probability of an error appearing on a bill is 0.6. Find the probability that on 10 bills we will identify 5 errors.
The probability of a specific 5 bills containing errors is (0.6^5)*(0.4^5)
There are ncr(10,5) ways of choosing those 5 bills ( ncr(10,5) = 10!/(5!*5!) )
so the overall probability is ncr(10,5)*(0.6^5)*(0.4^5)
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