The mole fraction of a substance in a mixture is the number of moles of that substance divided by the total number of moles of all substances present:

If a nonvolatile solute, A, is added to a pure solvent, B, until the mole fraction of the original solvent has decreased from one to X_B, then the vapor pressure will be only X_B times the vapor pressure of the pure liquid, p°_B

p_{B =} X_Bp° _B

This is Raoult's law. The lowering of vapor pressure of B will be proportional to the mole fraction of the added solute, A:

p_B=X_Ap°_B

You should be able to show that this follows from the previous expression, with the added fact that the sum of mole fractions is unity:

X_{A +} X_{B =}1