Let Vapour pressure of pure A be PAo
Let Vapour pressure of pure B be PBo
Using Raoult's law and Dalton's law:
\mathrm{When}{X}_{A}=0.7&{X}_{B}=0.3 {P}_{s}=350 \Rightarrow {P}_{A}^{o}\times 0.7+{P}_{B}^{o}\times 0.3=350....(i) \mathrm{When}{X}_{A}=0.2&{X}_{B}=0.8 {P}_{s}=410 \Rightarrow {P}_{A}^{o}\times 0.7+{P}_{B}^{o}\times 0.3=350....(\mathrm{ii}) \mathrm{Solving}(i)\mathrm{and}(\mathrm{ii}) {P}_{A}^{o}=314\mathrm{mm}\mathrm{Hg} {P}_{B}^{o}=434\mathrm{mm}\mathrm{Hg}