Answer :

Let p be the probability of the tube to function for more than 500 hours.


This probability is given as 0.2.


p = 0.2




If p is the probability of the tube to function for more than 500 hours, then q is the probability of the tube to not function for more than 500 hours.


p + q = 1


q = 1 – p





Let X denote a random variable that represents the number of the tube that can function for more than 500 hours out of the total 4 tubes.


And let n denote the total number of tube taken in the sample, that is, 4.


Then binomial distribution for r tube to function more than 500 hours out of 4 tubes is given by,


P (X = r) = nCrprqn-r


Putting n = 4, and above, we get



We need to find the probability that exactly 3 tubes will function for more than 500 hours.


So, put r = 3. We get









Thus, the probability that exactly 3 tubes will function for more than 500 hours is .


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