# Math Help - Counting-Combinations

1. ## Counting-Combinations

I am having problems understanding as to why n+r-1 C n-1= n+r-1 C r?

2. Originally Posted by qwerty10
I am having problems understanding as to why n+r-1 C n-1= n+r-1 C r?
Because $\dbinom{N}{k}=\dbinom{N}{N-k}$.

$(n+r-1)-(n-1)=r$