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}}$
$\Rightarrow$ ${P_2} = \frac{7}{{12}}{P_1} = \frac{7}{{12}} \times 20 = 11.67atm. \approx 11.7\,atm$