Solution:

Internal energy is \( U = n \frac{f}{2} RT \). For diatomic \( \text{O}_2 \) (rigid, \( f_1=5 \)), \( U_1 = 5 \times \frac{5}{2} RT = 12.5 RT \). For monoatomic \( \text{Ar} \) (\( f_2=3 \)), \( U_2 = 3 \times \frac{3}{2} RT = 4.5 RT \). Total \( U = 12.5RT + 4.5RT = 17RT \).