Hi

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 = .05 & q = .95 

nCx = 



a) P(exactly 3 of 20 being defective computers) = 1140(.05)^3(.95)17 = .0596

(b) P( test at least 5 computers to find 2 defective ones)

 P = 1 - (.05)^0*(.95)^5 + 5*(.05)^1*(.95)^4 = .0226  

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