Question

Water in a canal,30 dm wide and 12 dm deep,is flowing with a velocity of 20km/h.How much area will it irrigate if 9 cm of standing water is desired?

Answer 1

width of canal = 30 dm = 300 cm = 0.3 km
depth of canal =12 dm = 120 cm = 0.12 km
distance covered by canal in  30 mins = 20 x 30/60= 10 km 
amount of water irrigated  in 30 mins = 3/1000 x 12 /1000 x 10 =0.00036 km2
area irrigated in 0.009 km of standing water = 0.00036 / 0.009 =36x1000/9 x100000 =  0.4 km2

Answer 2

Answer:

Water in the canal forms a cuboid of breadth = 12 dm = 12/10 m = 1.2m , height = 30dm = 30/10 m = 3m

and,

$\sf{Length=distance\:covered\:by\:water\:in\:30\:minutes}$

$\sf{=Velocity\:of\:water×time}$

= $\sf{20000×}$$\sf\frac{30}{60}m=10000m$

________________________

$\therefore$ length of water flown in 30 min = lbh = (1.2 ×3 ×10000) m³ = 36000 m³

Suppose area irrigated be A m². Then,

$\sf{A×}$$\sf\frac{9}{100}=volume\:flown\:in\:30\:minutes$

=> $\sf{A×}$$\sf\frac{9}{100}=36000$

$\implies$ A = 400000 m²