Define power. $A$ boy of mass $45 \, kg$ climbs up $20$ steps in $20 \, s$. If each step is $25 \, cm$ high,calculate the power of the boy used in climbing. (Take $g = 10 \, m s^{-2}$)

(112.5 W) Power is defined as the rate of doing work.
Given:
Mass of the boy $(m)$ = $45 \, kg$
Number of steps $(n)$ = $20$
Height of each step = $25 \, cm = 0.25 \, m$
Total height $(h)$ = $20 \times 0.25 \, m = 5 \, m$
Time taken $(t)$ = $20 \, s$
Acceleration due to gravity $(g)$ = $10 \, m s^{-2}$
Work done $(W)$ = Potential energy gained = $mgh$
$W = 45 \times 10 \times 5 = 2250 \, J$
Power $(P)$ = $\frac{\text{Work}}{\text{Time}} = \frac{W}{t}$
$P = \frac{2250}{20} = 112.5 \, W$
Thus,the power of the boy is $112.5 \, W$.