For this question, we apply the Raoult's Law. The formula is written below:
P = P*x
where
P is the partial pressure
P* is the vapor pressure of the pure solvent
x is the mole fraction
The partial pressure is solved as follows:
P = Total P*x = (250 torr)(0.857) = 214.25 torr
Hence,
214.25 = (361 torr)(x)
x = 0.593 or 59.3%