The number of all possible combinations of 4 letters which are taken from the letters of the word 'ACCOMMODATION', is

If ${ }^n c_r=c_r$ and $2 \frac{c_1}{c_0}+4 \frac{c_2}{c_1}+6 \frac{c_3}{c_2}+\ldots .+2 n \frac{c_n}{c_{n-1}}=650$, then ${ }^n C_2=$ $\qquad$