The molar volume of liquid benzene (density = 0.877g/ml^-1) increases by a factor of 2750 as it vaporises at 20°C and that of liquid toluene (density = 0.867g mL^-1) increases by a factor of 7720 at 20°C. A solution of benzene and toluene at 20°C has a vapour pressure of 46.0 torr. Find the mole fraction of benzene in the vapour above the solution.