Let $V.P$ of pure $B$ be $P _B^0$

When $X _{ A }=0.7 \& X _{ B }=0.3$ $P _{ s }=350$

$\Rightarrow P _{ A }^0 \times 0.7+ P _{ B }^0 \times 0.3=350$

When $X _{ A }=0.2 \& X _{ B }=0.8$

$P _{ s }=410$

$\Rightarrow P _{ A }^0 \times 0.2+ P _{ B }^0 \times 0.8=410$

Solving $(i)$ and $(ii)$ $P _{ A }^0=314\,mm\,Hg$

$P _{ B }^0=434\,mm\,Hg$

$=(314)$