Explanation

(b) As $5\Omega$ resister is joined in parallel to series combination of $4\Omega$ and $6\Omega$ ($ie$, total resistance $10\Omega),\ V =$constant.
and $\frac{i_{1}}{i_{2}} = \frac{R_{2}}{R_{1}} = \frac{10}{5} = 2$
or $i_{2} = \frac{i_{1}}{2}$
Now heat produced per second in$5\Omega$ resistor
$H_{1} = i_{1}^{2}R_{1} = i_{1}^{2} \times 5 = 100Js^{- 1}$ …(i)
and for $4\Omega\ $resistor
$H_{2} = i_{2}^{2}R_{2} = \left( \frac{i_{1}}{2} \right)^{2} \times 4 = i_{1}^{2}$ …(ii)
Simplifying Eqs. (i) and (ii), we get
$\frac{H_{2}}{100} = \frac{1}{5}\ or\ H_{2} = \frac{1}{5} \times 100 = 20Js^{- 1}$