A gas mixture consists of 2 moles of oxygen and 4 moles of neon at temperature T. Neglecting all vibrational modes, the total internal energy of the system will be

$\text{Internal energy of }\mathrm{n}\text{ moles of gas is }\mathrm{U}=\frac{\mathrm{f}}{2}\mathrm{nRT}$ where f= degrees of freedom T= temperature of gas 
For mono atomic gas f=3
For diatomic gas f=5
Total internal energy of mixture of O₂ and Ne gases is
$\begin{array}{l}\mathrm{U}=\frac{{\mathrm{f}}_1}{2}{\mathrm{n}}_1\mathrm{RT}+\frac{{\mathrm{f}}_2}{2}{\mathrm{n}}_2\mathrm{RT}\\ \mathrm{U}=\frac{5}{2}\left(2\right)\mathrm{RT}+\frac{3}{2}\left(4\right)\mathrm{RT}\\ \mathrm{U}=11\mathrm{RT}\end{array}$