Two pumps are filling large vats of liquid
Rate 6 gallons per minute
Contains 28 gallons and is continuing to fill at rate of 4 gallons per minute
If two pumps are filling the large vats of liquid at a rate of 6 gallons per minute each, the total filling rate will be 6 gallons per minute + 6 gallons per minute = 12 gallons per minute.
If the large vat already contains 28 gallons and is continuing to fill at a rate of 4 gallons per minute, the total filling rate will be 12 gallons per minute + 4 gallons per minute = 16 gallons per minute.
Therefore, the large vat will be completely filled in (28 gallons / 16 gallons per minute) = 1.75 minutes.