Explanation

(b) Heat produced by the heater $H = \frac{V^{2}}{R}\ \times t$
For 220V heater heat produced
$\ \ \ \ \ \ \ \ \ \ \ \ \ \ \ \ \ \ \ \ \ \ \ \ \ \ \ H_{1} = \frac{(220)^{2}}{R}\ \times 5$
For 110V heater heat produced
$\ \ \ \ \ \ \ \ \ \ \ \ \ \ \ \ \ \ H_{2} = \frac{(110)^{2}}{R}\ \times L$
Now, $H_{1} = H_{2}$
$\ \ \ \ \frac{110 \times 110}{R}\ t = \frac{220 \times 220 \times 5}{R}$
$\ \ \ \ \ \ \ \ t = 20\ \min$