(c) $\mu = \mu_{1} + \mu_{2}$
$\frac{P(2V)}{RT_{1}} = \frac{P^{'}V}{RT_{1}} + \frac{P^{'}V}{RT_{2}} \Rightarrow \frac{2P}{RT_{1}} = \frac{P^{'}}{R}\left\lbrack \frac{T_{2} + T_{1}}{T_{1}T_{2}} \right\rbrack$
$P^{'} = \frac{2PT_{2}}{(T_{1} + T_{2})} = \frac{2 \times 1 \times 600}{(300 + 600)} = \frac{4}{3}atm$