# Math 20: Perms & Combs Help.

1. May 29, 2012

### dancing_math

1. The problem statement, all variables and given/known data

The expression nC5/nC4 can be simplified to:

2. Relevant equations

I'm assuming that I'm to use n!/(n-r)!r!

3. The attempt at a solution

I attempted to do this:
n!/(n-5)!5! / n!/(n-4)!4!
n!/(n-5)!5! x (n-4)!4!/n!
1/(n-5)!5! x (n-4)!4!
And I'm stuck after this point. The possible answers are:
A. 1/5(n-5)
B. (n-4)/5
C. 5(n-5)
D. none of the above.

2. May 29, 2012

### SammyS

Staff Emeritus
Hello dancing_math. Welcome to PF !

1/(n-5)!5! x (n-4)!4! is equivalent to $\displaystyle \frac{(n-4)!\,4!}{(n-5)!\,5!}\ .$

You should be able to further simplify $\displaystyle \frac{4!}{5!}$ and $\displaystyle \frac{(n-4)!}{(n-5)!}\ .$