At constant pressure heat absorbed \(=\Delta Q=n C_P \Delta T \quad \dots (1)\)
At constant volume heat absorbed \(=\Delta U=n C_v \Delta T \quad \dots (2)\)
Dividing \((1)\) by \((2)\),
\(\frac{\Delta Q}{\Delta U}=\frac{C_P}{C_V}=\gamma=1.4 \Rightarrow \frac{105}{\Delta U}=1.4\)
\(\therefore \Delta U_V=75 \,cal\)