Question 622155: A sample of 4 telephones is selected from a shipment of 20 phones. There are 5 defective telephones in the shipment. How many of the samples of 4 phones do not include any of the defective ones?
Answer by math-vortex(648)   (Show Source):
You can put this solution on YOUR website! Hi, there--
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This is a combinations problem. We want to choose samples with four non-defective phones and zero defective phones. In the shipment of 20 phones, five are defective. That means that 15 phones are non-defective.
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The total number of ways to choose four good non-defective phones is 15-choose-4, or 15C4.
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15C4 = 15! / (4! 11!)
15C4 = (15*14*13*12*11!) / (4*3*2*1*11!)
15C4 =1365
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There 1,365 ways to choose four working phones from the shipment.
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Hope this helps!
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Ms.Figgy
math.in.the.vortex@gmail.com
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