The outside of an ideal solenoid (N1 turns, length L, radius r) is wound with a coil

Question:

The outside of an ideal solenoid (N1 turns, length L, radius r) is wound with a coil of wire with N2 turns.
(a) What is the mutual inductance?
(b) If the current in the solenoid is changing at a rate ΔI1/Δt, what is the magnitude of the induced emf in the coil?