The coils having self-inductance of 10 mH and 15 mH have

The coils having self-inductance of 10 mH and 15 mH have an effective inductance of 40 mH, when connected in series aiding. What will be t he equivalent inductance if we connect them in series-opposing?

L = L₁ + L₂ ± 2M.
When ordinary, L = L₁ + L₂ + 2M
∴ 40 = 10 + 15 + 2M.
∴ 2M = 15 mH
When opposing, L = L₁ + L₂ – 2M
= 10 + 15 – 15 = 10 mH