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Permutation and combination

  1. Mar 23, 2012 #1
    1. The problem statement, all variables and given/known data
    Let Pn denotes the no. of ways of selecting 3 people out of 'n' sitting in a row if no two of them are
    consecutive and Qn is the corresponding figure when they are in circle .
    If Pn - Qn = 6 , then 'n' is equal to:



    3. The attempt at a solution

    Pn = (n-2) C 3
    Qn = (n-3) C 3
    But I am getting wrong answer.
    Please tell which one is wrong Pn or Qn .
     
  2. jcsd
  3. Mar 23, 2012 #2

    tiny-tim

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    Science Advisor
    Homework Helper

    hi nik jain! :smile:

    (try using the X2 and X2 buttons just above the Reply box :wink:)
    i'm mystified …

    why do you think that either of these is correct? :confused:

    but anyway you don't need to find Pn or Qn, you only need the difference

    what selections are in Pn but not in Qn ? :wink:
     
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