Water in a canal, 30 dm wide and 12 dm deep, is flowing with a velocity of 20 km per hour. How much area will it irrigate,

Width of the canal = 30 dm = 3 m                (1 m = 10 dm)

Depth of the canal = 12 dm = 1.2 m

Speed of the water flow = 20 km/h = 20000 m/h

∴ Volume of water flowing out of the canal in 1 h = 3 × 1.2 × 20000 = 72000 m3

Height of standing water on field = 9 cm = 0.09 m            (1 m = 100 cm)

Assume that water flows out of the canal for 1 h. Then,

Area of the field irrigated

$=\frac{\text { Volume of water flowing out of the canal }}{\text { Height of standing water on the field }}$

$=\frac{72000}{0.09}$

$=800000 \mathrm{~m}^{2}$

$=\frac{800000}{10000} \quad\left(1\right.$ hectare $\left.=10000 \mathrm{~m}^{2}\right)$

$=80$ hectare

Thus, the area of the field irrigated is 80 hectares.

Disclaimer: In this question time is not given, so the question is solved assuming that the water flows out of the canal for 1 hour.