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#1
01-31-2018, 07:21 PM
 Handynasty CAS SOA Join Date: Jan 2018 College: University of Virginia Posts: 11
Question needs explanation

Question:
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.

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!
#2
01-31-2018, 09:48 PM
 Gandalf Site Supporter Site Supporter SOA Join Date: Nov 2001 Location: Middle Earth Posts: 31,012

Why would you expect to use permutations? The order in which you get the items doesn't matter.

 Tags combinations, permutations