How many gallons of water does a 4' X 10' tank hold?

This cannot be answered as stated. There must be three dimensions given: width/depth/height. Or we must assume that the tank is "circular." More accurately, we assume the tank is a cylinder that is four feet high and 10 feet in diameter. The volume of a cylinder is given by the formula V = (Pi)r2h, where r is the radius of the base of the cylinder and h is its height. Since the radius is half the diameter, we get: V = (Pi) * (52) * 4 = 3.141 x 25 x 4 = 314.1 cubic feet of water. The question asked for the volume in gallons, however. One cubic foot of water contains about 7.48 gallons. So 314.1 x 7.48 = 2349.9 gallons.