This is a fairly straight-forward problem from A+ Click.
“The water from an open swimming pool evaporates at a rate of 5 gallons per hour in the shade and 15 gallons per hour in the sun. If the pool loses 8,400 gallons in June and there were no clouds, what is the average duration of night during that month?”
Answer Choices: 6 hours 8 hours 10 hours 12 hours
See Evaporating Pool Problem for solutions.