A 30 m deep well is having water upto 15 m. An engine evacuates it in one hour. The power of the engine, if the diameter of the well is 4 m is 

Mass of water, m = volume $\times \mathrm{density} = {\mathrm{\pi r}}^2\mathrm{l}\times \mathrm{\rho }$

= $\mathrm{\pi }{\left(2\right)}^2\times 15\times 1000 \mathrm{kg}$

height that the water is lifted is $\mathrm{h} = \frac{30+15}{2} = 22.5 \mathrm{m}$

$\mathrm{P} = \frac{\mathrm{W}}{\mathrm{t}} = \frac{\mathrm{mgh}}{\mathrm{t}} = \frac{\mathrm{\pi }{\left(2\right)}^2\times 15\times 1000\times 9.8\times 22.5}{3600}$

= 11.55 kW