Final answer:
It will take 8 more hours from 9:00 P.M. to fill the pond completely, so it will be full at 5:00 A.M.
Step-by-step explanation:
To determine at what time the pond will be full, we need to find the rate at which Burt is filling the pond.
From 9:00 A.M. to 9:00 P.M., a total of 12 hours have passed. During this time, Burt filled 2,100 gallons. So, his filling rate is 2,100 gallons / 12 hours = 175 gallons per hour.
To find the remaining amount of water needed to fill the pond, subtract the current water level (2,100 gallons) from the total capacity (3,500 gallons): 3,500 gallons - 2,100 gallons = 1,400 gallons.
Divide the remaining amount of water by the filling rate to determine how many hours it will take to fill the pond completely: 1,400 gallons ÷ 175 gallons per hour = 8 hours.
Therefore, the pond will be full at 9:00 P.M. + 8 hours = 5:00 A.M.