For adiabatic process: $TV^{\gamma-1} = \text{const}$
For monoatomic gas, $\gamma = \frac{5}{3}$, so $TV^{2/3} = \text{const}$
$$300 \cdot (2V)^{2/3} = T_2 \cdot (16V)^{2/3}$$
$$T_2 = 300 \cdot \left(\frac{2}{16}\right)^{2/3} = 300 \cdot \left(\frac{1}{8}\right)^{2/3} = 300 \cdot \frac{1}{4} = 75\,\text{K}$$