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There are two urns. One contains five white balls. The other contains four white balls and one black ball. An urn is selected at random and a ball in that urn is selected at random and removed. This procedure is repeated until one of the urns in empty. The probability that the black ball has not been selected is Binomial(10,5)/2^10. The form of the answer suggests a counting solution. What is being counted?

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Don't understand the question. Also doesn't sound like a research level question. –  supercooldave Jun 26 '10 at 12:52
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I am afraid, Martin, that you don't follow the idea of MO. Please read FAQ carefully to check whether your "problems" suit this site. –  Wadim Zudilin Jun 26 '10 at 13:31
    
See tea.mathoverflow.net/discussion/474/… –  Yemon Choi Jun 26 '10 at 19:54
    
Martin has reformulated this problem in a new question, see mathoverflow.net/questions/29807/binary-sequences-of-length-2n –  Scott Morrison Jun 28 '10 at 18:16
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closed as not a real question by Robin Chapman, Steve Huntsman, Harry Gindi, Wadim Zudilin, Greg Kuperberg Jun 26 '10 at 13:48

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