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Let the Area to be irrigated = A
height of water on area = 10cm = 0.1m
Volume of water to be irrigated = A×0.1
Volume of water flown in river in 1 hour= 3m × 1.2m × 20000 m/hr × 1hr = 72000m³

So A×0.1 = 72000
⇒A = 72000/0.1 = 720000 m²
⇒ A = 72 ha