The question, as stated, cannot be answered sensibly. A gallon is a measure of volume, with dimensions [L3]. A foot is a measure of distance, with dimensions [L]. The two measure different things and elementary dimensional analysis teaches that you cannot convert between measures with different dimensions such as these without additional information.
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You actually have a 20 gallon tank. There are 231 cubic inches in a gallon. So if you take your length times your with you get: 24x12 = 288in square. Then multiply that times the 16 inches of height and you get 4,608 inches cubed. Divide that number by the 231 cubic inches per gallon and you get: 4608 (inches cubed)/ 231(cubic inches per gallon)=19.94 Gallons or a 20 gallon tank.
400 square feet
65 616.7979 feet
20 yards in 60 feet
400 square feet.