A group of 9 students, s_1, ~s_2, , s_9 , is to be divided to from...

$$\begin{array}{ccc}\mathrm{x} & \mathrm{y} & \mathrm{z} \\ 2 & 3 & 4 \end{array}$$ C-i) when $x$ does not contain $\mathrm{S}_1$, but contains $\mathrm{S}_2$ (\underset{\text{(for }x)}{{ }^7 C_1} \times \underset{\text{(for }y,z)}{\frac{7!}{3!4!}}=245) C-ii) When $\mathrm{x}$ does not contain $\mathrm{S}_1, \mathrm{~S}_2$ and $\mathrm{y}$ does not contain $\mathrm{S}_2$ i.e. (\underset{\text{(for }x)}{{ }^{7} \mathrm{C}_2} \times \underset{\text{(for }y,z)}{\frac{6!}{3!3!}}=420) so total No. of ways $665$