Calculating n given r and nCr?

[moonman239]

Main Question or Discussion Point

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.

 

[mjpam]

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 $_{n}C_{r}$?

 

[chwala]

kindly read on combinations and permutations and be specific..

 

[HallsofIvy]

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.

 

[bpet]

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.