A solution has 1:4 molar ratio of pentane and hexane. The vapour pressure of pure hydrocarbons at 20^oC are 440 mm Hg for pentane and 120 mm of Hg for hexane. The mole fraction of pentane in vapour phase would be

mole fraction of pentane solution= 1/(1+4)=1/5
mole fraction of hexane solution=4/(1+4)= 4/5
the total pressure of solution,
P_s=X_pP'_p+X_HP'_H
=1/5 x 440 mm Hg + 4/5 x 120 mm Hg
=184mm Hg

The fraction of pentane in vapour phase= X_pP'_P/P_S =0.478

Hence option(B)=0.478 is correct.