A solid sphere of mass $M$ and radius $R$ is divided into two unequal parts. The smaller part having mass $\mathrm{M} / 8$ is converted into a sphere of radius r and the larger part is converted into a circular disc of thickness $t$ and radius $2 R$. If $I_1$ is moment of inertia of a sphere having radius $r$ about an axis through its centre and $\mathrm{I}_2$ is the moment of inertia of a disc about its diameter, the ratio of their moment of inertia $\mathrm{I}_2 / \mathrm{I}_1=$ $\_\_\_\_$ .