Two liquids A and B have ${\mathrm{P}}_{\mathrm{A}}^{\mathrm{o}}$ and ${\mathrm{P}}_{\mathrm{B}}^{\mathrm{o}}$ the ratio of 1 : 3. If the ratio of number of moles of A and B are 1 : 3, the mole fraction of ‘A’ in vapour phase in equilibrium with the solution is equal to –