Explanation

(c) Let for mixture of gases, specific heat at constant volume be $C_{V}$
$C_{V} = \frac{n_{1}\left( C_{V} \right)_{1} + n_{2}\left( C_{V} \right)_{2}}{n_{1} + n_{2}}$
where for oxygen; ${C_{V}}_{1} = \frac{5R}{2},\ n_{1} = 2\ mol$
For helium; ${C_{V}}_{2} = \frac{3R}{2},\ n_{2} = 8\ mol$
Therefore, $C_{V} = \frac{\frac{2 \times 5R}{2} + 8 \times \frac{3R}{2}}{2 + 8} = \frac{17R}{10} = 1.7\ R$