Measure coupling by reversing one coil

Two coils with separately measured self-inductances 20 mH and 30 mH stay fixed in space. Their series inductance is 70 mH in one connection and 30 mH after the terminals of one coil are reversed.

What is the magnitude of their coupling coefficient?

Organize the givens & assumptions

L₁ = 20 mH; L₂ = 30 mH
Series measurements: 70 mH and 30 mH

Linear reciprocal coils, unchanged geometry, same frequency regime for every measurement, negligible parasitic capacitance. Only the terminal connection is reversed.

A $1/\sqrt6$
B $1/4$
C $1/2$
D $1$

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