How many gallons of fluid are in a cylindrical fuel tank that is 7 feet diameter 20 feet long?

There will be about 5757 US gallons in a full tank of this size. A cylinder's volume is the area of the base times its height. The area of the base is pi times the square of the radius of that base. A radius is half a diameter. Half of 7 feet is 3 1/2 feet. If we take pi times the square of that, we'll get about 38.48 square feet. Multiply that by the 20 foot length and you'll get about 769.6 cubic feet. There are about 7.5 gallons in a cubic foot of water. Multiply the 769.6 feet3 (the volume) by 7.5 gallons/foot3 and there's the (approximately) 5757 gallons (US) in that tank.