Mass of benzene = 30
molar mass of benzene = 78
moles of benzene = 30/78
= 0.38
mass of carbon tetrachloride = 70
molar mass of carbon tetrachloride = 154
moles of carbon tetrachloride = 70/154
= 0.45
total number of moles = 0.38 + 0.45 = 0.83
mole fraction of benzene = 0.38 / 0.83
= 0.45