Two circular coils have their centers at the same point. The mutual inductance between them will be maximum when their axes

If a change in current of $0.01 \,A$ in one coil produces a change in magnetic flux of $2 \times 10^{-2} \,Wb$ in the other coil,then the mutual inductance of the two coils in henry is (in $\,H$)

What is the coefficient of mutual inductance when the magnetic flux changes by $2 \times 10^{-2} \, Wb$ and the change in current is $0.01 \, A$?

If a change in current of $0.01\, A$ in one coil produces a change in magnetic flux of $1.2 \times 10^{-2}\,Wb$ in the other coil,then the mutual inductance of the two coils in henries is.....$H$

Alternating current of peak value $(\frac{2}{\pi}) \ A$ flows through the primary coil of a transformer. The coefficient of mutual inductance between the primary and secondary coil is $1 \ H$. The peak e.m.f. induced in the secondary coil is (Frequency of a.c. $= 50 \ Hz$) (in $V$)

$A$ circular coil of radius $r$ is placed on another circular coil whose radius is $R$. The current flowing through the larger coil is changing, and their centers coincide. Given $R \gg r$, if both coils are coplanar, then the mutual inductance between them is proportional to: