Permutation and combination

  • Thread starter nik jain
  • Start date
  • #1
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Homework Statement


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:



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 .
 

Answers and Replies

  • #2
tiny-tim
Science Advisor
Homework Helper
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hi nik jain! :smile:

(try using the X2 and X2 buttons just above the Reply box :wink:)
Pn = (n-2) C 3
Qn = (n-3) C 3
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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