EMF $\varepsilon = M \large\frac{dI}{dt}$

Given $\varepsilon = 50 mV = 50 \times 10^{−3}V$, $I_1 = 4A, I_2 = 8A$ and $ dt = 0.5s$,

$\Rightarrow M = \varepsilon \large\frac{dt}{dI} $$ = 50 \times 10^{-3} \large\frac{0.5}{8 - 4}$ $= - 6.25 \times 10^{−3}$