Electromagnetic Induction

Two planar concentric rings of metal wire having radii $\mathrm{r}_1$ and $\mathrm{r}_2\left(\mathrm{r}_1>\mathrm{r}_2\right)$ are placed in air. The current I is flowing through the coil of larger radius. The mutual inductance between the coils is given by ( $\mu_0=$ permeability of free space)