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Homework Help: Probability Question

  1. Oct 2, 2008 #1
    There are n objects, within which there are p identical objects. Now, suppose if you randomly choose r objects from the n objects. How many permutations will you get from the r objects you choose?
     
  2. jcsd
  3. Oct 3, 2008 #2
    So we have n balls, p are identical. Are the rest not identical? I.e. are exactly p identical or at least p identical?

    Suppose it's exactly p. One way is to do it by cases. Choose k from the n-p distinct objects and r-k from the p identical. In that case you have a sum over k=0...r.

    Now imagine r identical balls, choose r-k to remain identical. Now paint the remaining k with distinct numbers from 1 to n-p. How many possibilities?
     
  4. Oct 5, 2008 #3

    HallsofIvy

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    Science Advisor

    Your question is ambiguous. Are there n containers each with p objects? And the objects are identical only within each container- different containers contain different objects? Or are there a total of p (or np?) identical objects?
     
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