Answer: 33 gallons
Step-by-step explanation:
The water tank is a cube, with one dimension of x meters. Its volume will be x^3.
X^3 = 729m^3
x = 9 meters. The cube has 4 side walls and a top and bottom, for a total of 6 interior walls. Each wall has an area of x^2, or 81 m^2.
(6 walls)*(81 ft^2) = 486 m^2
One gallon of paint will cover 15 m^2. The conversion factor is (1 gallon/15 m^2)
(486 m^2)*(1 gallon/15 m^2) = 32.4 gallons, rounded up to 33 gallons