(c) $PV = \mu RT = \frac{m}{M}RT \Rightarrow P \propto mT$
$\Rightarrow \frac{P_{2}}{P_{1}} = \frac{m_{2}}{m_{1}}\frac{T_{2}}{T_{1}} = \frac{1}{2} \times \frac{(273 + 27 + 50)}{(273 + 27)} = \frac{7}{12}$
$\Rightarrow P_{2} = \frac{7}{12}P_{1} = \frac{7}{12} \times 20 = 11.67atm.\ \approx 11.7atm$