X, Y, and Z are three volatile liquids, which when mixed make an ideal solution. At a given temperature $\text{T}_0$, the constituents X, Y, and Z have pure vapour pressures in the ratio 3:2:1. A mixture prepared with mole-fractions of X, Y, and Z in the ratio 1:2:3 starts to boil at temperature $\text{T}_0$ at 1.5 atm pressure. At temperature $\text{T}_0$, the partial vapour pressure (in atm) of Y is: