Discussion
Two coils have self-inductance of 20 H and 20 H connected in series and mutual inductance between two coils is 2.5 H. The value of total equivalent inductance is:
Answer: B
When two coils are connected in series so that their fluxes aid, the total inductance is the sum of the self inductances plus twice the mutual inductance.
\[ \begin{aligned} L = L_1 + L_2 + 2M = 20 + 20 + 2 \times 2.5 \\ &= 45\ \text{H} \end{aligned} \]
The item does not say that the coils oppose, so the aiding connection is taken, the usual default. With opposing fluxes the result would be 35 H, which is not offered. The value 42.5 H would be \(L_1 + L_2 + M\), and 40 H ignores the coupling. The answer is 45 H, option B.
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