In a mixture of A and B, if the mole fraction of the component A in vapour phase is $X_1$ and mole fraction of component A in liquid mixture is $X_2 ( P_A^0$= vapour pressure of pure A; $P_B^0$ = vapour pressure of pure B), then total vapour pressure of the liquid mixture is :

$\large\frac{P_A^0X_2}{X_1}$