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