$\left(C_{v}\right)_{\text {mix }} =\frac{n_{1} c_{v_{1}}+n_{2} C_{v_{2}}}{n_{1}+n_{2}}\left[\begin{array}{c} \text { In this question, } n_{1}=n_{2}=n \\ c_{v_{1}}=\frac{5 R}{2}, c_{v_{2}}=5 R / 2 \end{array}\right]$

$=\frac{n\left(\frac{5 R}{2}\right)+n \cdot\left(\frac{5 R}{2}\right)}{2 n}=\frac{5 R}{2}:$

$\left(C_{P}\right)_{\text {mix }} =\frac{5 R}{2}+R=\frac{5 R+2 R}{2}=\frac{7 R}{2}$

$\therefore Ratio=7 / 5$

$\text { Correct choice }$