Explanation

(a) Power is the amount of work done or energy $(Q)$ transferred per unit of time $(t).$
$\therefore\ \ \ P = \frac{H}{t}$
$H = mc\ \mathrm{\Delta}\theta$
Where $m$ is mass, $c$ the specific heat and $\mathrm{\Delta}\theta$ the temperature difference.
$\therefore P = \frac{mc\ \mathrm{\Delta}\theta}{t}$
$\mathrm{\Delta}\theta = (35 - 20)℃ = 15℃$
$C = 1000,\ 4.2\ J = 1\ cal$
$t = 1\min{= 60s}$
$\therefore P = \frac{1 \times 1000 \times 15 \times 4.2}{60}\ {Js}^{- 1} = 1050\ W$