Let V.P. of pure A be PA0 Let V.P of pure B be PB0 When XA=0.7&XB=0.3 ‌Ps=350 ‌⇒PA0×0.7+PB0×0.3=350 . . . (i) When XA=0.2&XB=0.8 ‌Ps=410 ‌⇒PA0×0.2+PB0×0.8=410 . . . (ii) Solving (i) and (ii) ‌PA0=314‌mm‌Hg ‌PB0=434‌mm‌Hg ‌=(314)