Hi,

From a lot of 12 missiles, 5 are selected at random and fired. 

If the lot contains 3 defective missiles th are defective

P(5 will fire) = (3/4)^5

P(at most 2 will fire) = P(0 fire) + P(1 fire) + P(2 fire)

                       = (1/4)^5 + 5(1/4)^4(3/4)^1 + 10(1/4)^3(3/4)^2



Note: The probability of x successes in n trials is: 

P = nCx*  where p and q are the probabilities of success and failure respectively. 

In this case p = 3/4 & q = 1/4

nCx = 

 

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