# Math Help - combinations

1. ## combinations

we want to up stairs with 12 steps. We can pass by to steps on once. How many ways can we up the stairs?
Solution: 233

The number of ways we can do that is $\frac{9!}{3!\cdot 6!}=\binom{9}{3}$.
So calculate $\sum\limits_{k = 0}^6 \binom{12-k}{k}$