Calculating n given r and nCr?

  • Thread starter moonman239
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  • #1
moonman239
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I wish to calculate the number of objects in the population I'm selecting from, given that I am choosing r objects and there are nCr different combinations.
 

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  • #2
mjpam
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I wish to calculate the number of objects in the population I'm selecting from, given that I am choosing r objects and there are nCr different combinations.

Do you know the equation for [itex]_{n}C_{r}[/itex]?
 
  • #3
chwala
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kindly read on combinations and permutations and be specific..
 
  • #4
HallsofIvy
Science Advisor
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You can do it for reasonable values of C and r but there is no simple formula- it is really a matter of factoring as you can the specific value of C. And, of course, it is important that C actually be a binomial coefficient. The great majority of integers are NOT.
 
  • #5
bpet
530
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It can be done by a simple search starting at n=r and using (n+1)Cr = nCr*(n+1)/(n+1-r). Since the terms are increasing it is guaranteed to find a solution if it exists.
 

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