what is mutual induction in the following question?

Question

Two coils are placed close together in a physics lab to demonstrate Faraday’s law of induction. A current of 5.00 A in one is switched off in 1.00 ms, inducing a 9.00 V emf in the other. What is their mutual inductance?

Answer 1

The voltage produced in coil 2 is:

V₂ = -d/dt (N₂Φ₂₁)

= -d/dt (MI₁)

V₂ = -M * dI₁/dt

Which means dI₁ / dt ≈ ΔI₁/Δt if our time lapse is finite

M = -V₂Δt / ΔI₁

= -(9 V)(1e-3 s) / (5 A)

= 1.8e-3