SOLUTION: In a shipment of 20 smartphones, 2 are defective. How many ways can a quality control inspector randomly test 4 smartphones, of which 2 are defective?

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Question 1173046: In a shipment of 20 smartphones, 2 are defective. How many ways can a quality control inspector randomly test 4 smartphones, of which 2 are defective?
Answer by ikleyn(52803)   (Show Source): You can put this solution on YOUR website!
.

Every time, the inspector adds 2 good smartphones to 2 defective.


He can select 2 good smartphones from the 18 good smartphones in   ways =  = 9*17 = 153 ways.


It is the number of all possible favorable quadruples.


The number of all possible quadruples is, meanwhile,   =  = 4845.


THEREFORE, the probability under the problem's question is


    P =  =  =  =  = 0.03157895.    ANSWER

Solved.


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