Equal volumes of two monoatomic gases, $A$ and $B$, at same temperature and pressure are mixed. The ratio of specific heats ($C_p/C_v$) of the mixture will be:

For monoatomic gases:$C_v=\frac{3}{2}R$ and $C_p=\frac{5}{2}R$.The ratio $\gamma =\frac{C_p}{C_v}=\frac{5/2R}{3/2R}\approx 1.67$.When two monoatomic gases are mixed, the mixture remains a collection of monoatomic particles. The molar heat capacities of the mixture will be the same as the individual gases.Therefore, ${\gamma }_{mix}=1.67$.