# Pre- calc HELP! Combinations

• Jul 28th 2013, 12:27 AM
x83
Pre- calc HELP! Combinations
Hi there the question states 2CR=25 I am really new to this so any help would be appreciated it states to solve for R.

I tried to solve but got no where what i tried was to expand with the formula of nCr and no luck :(
• Jul 28th 2013, 12:49 AM
Re: Pre- calc HELP! Combinations
2 choose R = 25 is not possible.
• Jul 28th 2013, 12:50 AM
x83
Re: Pre- calc HELP! Combinations
Alright

But if it was, what is the process of solving one of these guys?
• Jul 29th 2013, 02:50 AM
emakarov
Re: Pre- calc HELP! Combinations
Quote:

Originally Posted by x83
But if it was, what is the process of solving one of these guys?

For small values of n, I would search the Pascal's triangle.
• Jul 29th 2013, 03:08 AM
Plato
Re: Pre- calc HELP! Combinations
Quote:

Originally Posted by x83
Hi there the question states 2CR=25 I am really new to this so any help would be appreciated it states to solve for R. I tried to solve but got no where what i tried was to expand with the formula of nCr and no luck :(

Using the notation $_NC_k$ unless you are in an advanced mathematics class it is necessary that $N\ge k.$

If that is true then $_NC_k=M$ implies $\frac{N!}{k!(N-k)!}=M$.

Example: $_{N}C_2=105$ implies $\frac{N(N-1)}{2}=105$, solve for N.