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 January 17th, 2011, 07:37 AM #1 Member   Joined: Oct 2010 Posts: 37 Thanks: 0 permutation problem A non-empty P is formed by selecting elements randomly WOR from a set B consisting of n (>1) distinct elements. Another non-empty subset Q is formed in similar fashion from the original set B consisting of those same n elements. Then what is the probability that P and Q do not have any element common between them?
 January 17th, 2011, 01:08 PM #2 Global Moderator   Joined: May 2007 Posts: 6,821 Thanks: 723 Re: permutation problem Assume P has k elements and Q has j elements, the probability of nothing in common is: (n-j)!(n-k)!/[n!(n-j-k)!] Note that j+k?n, otherwise the denominator is ? and the probability = 0. Exercise for the reader - derive this expression!

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