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 May 10th, 2010, 09:18 PM #1 Newbie   Joined: May 2010 Posts: 1 Thanks: 0 Probability Problem.Please help A lot contains 20 articles.The probability that the lot contains exactly 2 defective articles is 0.4 and that it contains exactly 3 defective articles is 0.6.Articles are drawn from the lot at random 1 by 1 without replacement and tested till all the defective articles are found.What is the probability that this procedure ends at the 12th testing?
 May 11th, 2010, 06:46 AM #2 Senior Member   Joined: Apr 2008 Posts: 435 Thanks: 0 Re: Probability Problem.Please help I have a small question. Presume, for a moment, that we happen to choose a lot which has 2 defective articles. Then, as we don't know whether we're in a lot with 2 or 3 defective articles, we must test all 20 before we realize that there is no third defective article... unless I am missing an assumption.

### a lot contains 20 articles. 2 defective. what is the probability ends at 12th testing

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