(a) Area of the circle swept by the windmill = A
Velocity of the wind = v
Density of air = ρ
Volume of the wind flowing through the windmill per sec = Av
Mass of the wind flowing through the windmill per sec = ρAv

Mass m, of the wind flowing through the windmill in time t = ρAvt
(b) Kinetic energy of air $=\frac{1}{2}m{v}^2$

$=\frac{1}{2}\left(\mathrm{\rho Avt}\right){\mathrm{v}}^2=\frac{1}{2}{\mathrm{\rho Av}}^3\mathrm{t}$
(c) Area of the circle swept by the windmill = A = 30 ${\mathrm{m}}^2$
Velocity of the wind = v = 36 km/h
Density of air, ρ = 1.2 kg ${\mathrm{m}}^{-3}$
Electric energy produced = 25% of the wind energy

$\begin{array}{l}=\frac{25}{100}\times \text{ Kinetic energy of air }\\ =\frac{1}{8}{\mathrm{\rho Av}}^3\mathrm{t}\end{array}$
$\begin{array}{l}\text{ Electrical power }=\frac{\text{ Electrical energy }}{\text{ Time }}\\ =\frac{1}{8}\frac{{\mathrm{\rho Av}}^3\mathrm{t}}{\mathrm{t}}=\frac{1}{8}{\mathrm{\rho Av}}^3\\ =\frac{1}{8}\times 1.2\times 30\times (10{)}^3\\ =4.5\times {10}^3\mathrm{W}=4.5kW\end{array}$