Question 967414
18 phones total
16 nondefective
2 defective


Testing 4 phones: 2 defective, 2 nondefective


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Number of ways to pick 2 defective phones:


2 C 2 = (2!)/(2!*(2-2)!) = 1


I'm using the <a href="http://www.mathwords.com/c/combination_formula.htm">combination formula</a>.


There is 1 way to pick 2 defective phones from a pool of 2 total defective phones.

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Number of ways to pick 2 nondefective phones


16 C 2 = (16!)/(2!*(16-2)!) = 120


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So there are 1*120 = 120 different ways to pick 4 smart phones, of which 2 are defective