Answer:
484 hours
Explanation:
The given quantity of water will be added in a time period equal to the quantity divided by the rate.
(800 ac·ft)(43560 ft²/ac)/(20 ft³/s × 3600 s/h) = 484 h
It will take 484 hours to raise the water level by 1 ft.
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Additional comment
We have assumed the given rate is a net rate of water influx. Evaporation from 800 acres may amount to about 1.3 ft³/s, or 6.6% of the inflow rate. It could be more than 3 times that amount, depending on the climate conditions at the reservoir's location.
