Area = A

Velocity $=\mathrm{V}$

Density $=\rho$

(a) Volume of the wind through the windmill per sec is given by $=\mathrm{Av}$

Mass is given by $=\rho \mathrm{AV}$

So,

Mass $m$ through the windmill in time $t$ will be $\rho Avt$

(b) kinetic energy is given by the relation $\frac{1}{2} \mathrm{mv}^{2}$

$=\frac{1}{2}(\rho \mathrm{Avt}) \mathrm{v}^{2}=\frac{1}{2} \rho \mathrm{Av}^{3} \mathrm{t}$

(c) Area is given as $30 \mathrm{~m}^{2}$

Velocity is given as $36 \mathrm{~km} / \mathrm{h}$

Density of air si given as $\rho=1.2 \mathrm{~kg} \mathrm{~m}^{-3}$

Electric energy is $25 \%$ of wind energy

$=\frac{25}{100} \mathrm{x}$ kinetic energy

$=\frac{1}{8} \rho \mathrm{Av}^{3} \mathrm{t}$

Power $=\frac{\text { Electric energy }}{\text { Time }}$

$=\frac{1}{8} \frac{\rho A v^{3} t}{t}=\frac{1}{8} \quad \rho \mathrm{Av}^{3}$

$=\frac{1}{8} \times 1.2 \times 30 \times 10^{3}$

$=4.5 \times 10^{3} \mathrm{~W}=4.5 \mathrm{~kW}$