A gas mixture consists of 4 mol of oxygen and 6 mol of argon at temperature T. Neglecting all vibrational
modes, the total internal energy of the system is

Oxygen is diatomic gas, hence its energy for four moles

$=4\times \frac{5}{2}RT=10RT$

Argon is a monoatomic gas, hence its internal energy for

$6\mathrm{mol}=6\times (3/2)RT=9RT$

Total internal energy  $=(9+10)RT=19RT$