A substance ' X ' ( 1.5 g ) dissolved in 150 g of a solvent ' Y ' (molar mass $=300 \mathrm{~g} \mathrm{~mol}^{-1}$ ) led to an elevation of the boiling point by 0.5 K . The relative lowering in the vapour pressure of the solvent ' Y ' is $\_\_\_\_$ $\times 10^{-2}$. (Nearest integer)$\\$ [Given : $\mathrm{K}_{\mathrm{b}}$ of the solvent $=5.0 \mathrm{~K} \mathrm{~kg} \mathrm{~mol}^{-1} ]\\$ Assume the solution to be dilute and no association or dissociation of X takes place in solution.