A mixture of ideal gases $7kg$ of nitrogen and $\text{11\hspace{0.17em}}kg$ of $C{O}_2$ . Then 

(Take $\gamma$ for nitrogen and $C{O}_2$ as 1.4 and 1.3 respectively)

equivalent molecular weight of the mixture is 36

equivalent molecular weight of the mixture is 18

$\gamma$ for the mixture is $5/2$

$\gamma$ for the mixture is $47/35$