Two identical spheres each of mass $2 \mathrm{~kg}$ and radius $50 \mathrm{~cm}$ are fixed at the ends of a light rod so that the separation between the centers is $150 \mathrm{~cm}$. Then, moment of inertia of the system about an axis perpendicular to the rod and passing through its middle point is $\frac{x}{20} \mathrm{~kg} \mathrm{m^{2 }}$, where the value of $x$ is ___________.