# Number of ways to climb 11 stairs if you can climb 1 or 2 stairs every time

• April 29th 2013, 10:00 AM
verasi
Number of ways to climb 11 stairs if you can climb 1 or 2 stairs every time
Hi,

I have a math unit test on combinatorics and I couldn't solve this problem. Please help!

• April 29th 2013, 10:54 AM
Plato
Re: Number of ways to climb 11 stairs if you can climb 1 or 2 stairs every time
Quote:

Originally Posted by verasi
I have a math unit test on combinatorics and I couldn't solve this problem. Please help!

Here are the possibles:
$\begin{array}{*{20}{c}}5&{twos}&{}&1&{one}\\4&{two s}&{}&3&{ones}\\3&{twos}&{}&5&{ones}\\2&{twos}&{}& 7&{ones}\\1&{two}&{}&9&{ones}\\0&{twos}&{}&{11}&{o nes}\end{array}$

For example, we can arrange 4 twos and 3 ones in $\frac{7!}{4!\cdot 3!}$ ways.

Now you do each and add.