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Old 01-31-2018, 06:21 PM
Handynasty Handynasty is offline
Join Date: Jan 2018
College: University of Virginia
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Post Question needs explanation

A store has 80 modems in its inventory, 30 coming from Source A and the
remainder from Source B: Of the modems from Source A; 20% are defective.
Of the modems from Source B; 8% are defective.
Calculate the probability that exactly two out of a random sample of 5
modems from the store's inventory are defective.

The official answer is:
There are total of 10 defects.
The probability is (10choose2)*(70choose3)/(80choose5)

I don't understand why we are using combinations instead of permutations. My answer was (10P2)*(70P3)/(80P5). Hope someone can enlighten men. Thanks!
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Old 01-31-2018, 08:48 PM
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Gandalf Gandalf is offline
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Why would you expect to use permutations? The order in which you get the items doesn't matter.
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combinations, permutations

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