
#1




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. 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! 
Tags 
combinations, permutations 
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