A short solenoid of radius a and n1 number of turns per unit length and length L is kept coaxially inside a very long solenoid of radius b and n2 number of turns per unit length. What is the mutual inductance of the system.

µ0πb2n1n2L µ0πa2n1n2L2 µ0πa2n1n2L µ0πb2n1n2L2

µ0πa2n1n2L

$ M = \mu_0 n_1 n_2 \pi a^2 L$