GATE 2023 AR — Planning – Question 44
A building has 500 m² roof area. Annual rainfall is 0.4 m and 40% of collected roof rainwater is lost. If daily demand is 600 litres, for how many days can the annual usable harvest meet demand? Give an integer.
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Correct answer: 200 to 200
Explanation
Gross harvest is 500×0.4=200 m³. After 40% loss, usable water is 200×0.60=120 m³=120000 litres. Divide by 600 litres/day to obtain 200 days. Apply the retained fraction 0.60 rather than treating the loss as usable water.