If you sprint up a staircase, you consume more power than if you walk slowly, because you are doing the same amount of total work (lifting your weight to the same height) in a much shorter period.

Power is the 'rate' of the job: P = Work over Time (P=W/T).

The mechanical work done against gravity is calculated as $W = m \times g \times h$. Since the weight ($W_{g}$) of the boy is given as $m \times g = 600 \, \text{N}$, the work done is $600 \, \text{N} \times 10 \, \text{m} = 6000 \, \text{J}$. Power is then calculated using the formula $P = \frac{W}{t}$, where $t = 20 \, \text{s}$, giving $P = \frac{6000}{20} = 300 \, \text{W}$.

In competitive exams, always check units first; if weight is given in Newtons, you do not need to multiply by $g (9.8 \, \text{m/s}^2)$, as it is already included.