The mutual inductance between two planar concentric rings of radii r1 and r2 (with r1>>r2) placed in air is given by
μ0πr222r1
Magnetic field due to the larger coil at its centre is
B=μ0I2r1 where I is the current in the larger coil. Flux through the inner coil is
ϕ=B×πr22=μ0I2r1×πr22
But ϕ=MI. Therefore
M=μ0πr222r1