(i) Definition of mutual inductance
(ii) Calculation of change of flux linkage
(i) Magnetic flux, linked with the secondary coil due to the unit current flowing in the primary coil, ${\mathit{\varphi }}_2=\mathit{M}{\mathit{I}}_1$
[Alternatively
Induced emf associated with the secondary coil, for a unit rate of change of current in the primary coil. ${\mathit{e}}_2=-\mathit{M}\frac{\mathit{d}{\mathit{l}}_1}{\mathit{d}\mathit{t}}$]
[Also accept the definition of Mutual Induction, as per the Hindi translation of the question]
[i.e. the phenomenon of production of induced emf in one coil due to change in current in neighbouring coil]
(ii) Change of flux linkage
$\mathit{d}\mathit{\varphi }=\mathit{M}\mathit{d}\mathit{l}$
$=1.5\times \left(20-0\right)\mathit{W}$