c
(c) $\mu = {\mu _1} + {\mu _2}$
$\frac{{P(2V)}}{{R\,{T_1}}} = \frac{{P'v}}{{R\,{T_1}}} + \frac{{P'V}}{{R\,{T_2}}}$==> $\frac{{2P}}{{R\,{T_1}}} = \frac{{P'}}{R}\left[ {\frac{{{T_2} + {T_1}}}{{{T_1}\;{T_2}}}} \right]$
$P' = \frac{{2P\,{T_2}}}{{({T_1} + {T_2})}} = \frac{{2 \times 1 \times 600}}{{(300 + 600)}} = \frac{4}{3}atm$