In the case of miscible liquids (solution of a liquid in liquid), the partial vapour pressure exerted by each components is proportional to its molar concentration in the mixture solution. The total vapour pressure P is given by:

P=PA+PB=PoAXA+PoBXB

where,
XA and XB are mole fractions of components

A and B respectively
PoA & PoB are the vapor pressure exerted by the pure components
A and B respectively.