starting with 12,500 gallons, if the hose was running for three hours, 180 gallons would have been added to the pool. 12,500+180=12,680. if the valve was open for six hours, (20x6=120) 120 gallons would have been lost. 12,680-120=12,560. 12,560 gallons of water are in the pool at the end of the day.