An ideal solenoid (N1 turns, length L1, radius r1) is placed inside another ideal solenoid (N2 turns,

Question:

An ideal solenoid (N1 turns, length L1, radius r1) is placed inside another ideal solenoid (N2 turns, length L2 > L1, radius r2 > r1) such that the axes of the two coincide.
(a) What is the mutual inductance?
(b) If the current in the outer solenoid is changing at a rate ΔI2/Δt, what is the magnitude of the induced emf in the inner solenoid?