A conducting sphere with inner radius R and outer radius 3R has some alloys mixed in it due to which its resistivity $\left(\rho \right)$ changes with radial distance $r(>R)$ according to the function $\rho =Kr$, where $K$ is +ve constant. Inner surface of the sphere is grounded while outer surface of sphere in maintained at potential  $V_0$ with the help of an ideal battery of emf  $V_0$.  The potential of the sphere at radial distance of 2R is  $V_0\frac{\ln a}{\ln b}$. ($a, b=$least possible natural numbers). Then value of  $\frac{a}{b}$ is