(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, $\phi_2 = M I_1$[Alternatively Induced emf associated with the secondary coil, for a unit rate of change of current in the primary coil. $e_2 = -M \;\frac{d l_1}{d t}$][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 $d\phi \;= M d l$ $\;=1.5 \times (20-0) \mathrm{W}$ $\;=30 \;\text{ weber }\;$