p_A and p_B are the vapour pressure of pure liquid components, A and B, respectively of an ideal binary solution.
If X_A represents the mole fraction of component A, the total pressure of the solution will be:

1. p_A + X_A (p_B-p_A)
2. p_A + X_A (p_A-p_B)
3. p_B + X_A (p_B-p_A)
4. p_B + X_A (p_A-p_B)

A solution has a 1:4 mole ratio of pentane to hexane.
The vapor pressures of the pure hydrocarbons at 20 ºC are
440 mm Hg for pentane and 120 mm Hg for hexane.
The mole fraction of pentane in the vapor phase would be: