Elevation in boiling point for 1.5 molal solution of glucose in water is 4 K . The depression in freezing point for 4.5 molal solution of glucose in water is 4 K . The ratio of molal elevation constant to molal depression constant $\left(\mathrm{Kb}_{\mathrm{b}} / \mathrm{K}_{\mathrm{f}}\right)$ is $\_\_\_\_$ .