If 2 mol of an ideal monoatomic gas at temperature T_o are mixed with 4 mol of another ideal monoatomic gas at temperature 2T_o, then the temperature of the mixture is : 

\(\frac{5}{3}T_o\)

Let T be the temperature of the mixture.

Then U = U₁ + U₂

\(\Rightarrow \frac{f}{2} (n_1 + n_2) RT = \frac{f}{2}n RT_o + \frac{f}{2} nR(2T_o)\)

(2 + 4)T = 2T_o + 8T_o  ... [because n₁ = 2, n₂ = 4]

T = \(\frac{5}{3}T_o\)