The densities of two solid spheres A and B of the same radii R vary with radial distance r as $${\rho _A}(r) = k\left( {{r \over R}} \right)$$ and $${\rho _B}(r) = k{\left( {{r \over R}} \right)^5}$$, , respectively, where k is a constant. The moments of inertia of the individual spheres about axes passing through their centres are ${I_A}$ and ${I_B}$, respectively. If, ${{{I_B}} \over {{I_A}}} = {n \over {10}}$, the value of n is