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May 10th, 2010, 10:18 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, 07:46 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.


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