$\therefore T _{ b }^{0}+\Delta T _{ b }-\left( T _{ f }^{0}-\Delta T _{ f }\right)=75.7$

$\therefore 80+\Delta T _{ b }-\left(5.5-\Delta T _{ f }\right)=75.7$

$\quad \therefore \Delta T _{ b }+\Delta T _{ f }=1.2$

$\therefore \quad imKb + imKf =1.2$

$\quad \therefore i =\frac{1.2}{ m ( Kb + Kf )}$

$=\frac{1.2}{0.2(2.65+5.12)}$

$\quad \therefore i =0.77$