Math CentralQuandaries & Queries


Question from Marie, a student:

Hi can you help me solve this problem. the answer is 4.1 x 10^5 but, I do not know how to solve it.

A water treatment plant is built with three cylindrical tanks to contaim the water for a town. Each tank has a radius of 15 feet and a depth of 25 ft.

If there are about 7.5 gallons in a cubic foot of water, approximately how many gallons of water can be treated at the plant at any one time?


Hi Marie. To make things simple, we can think of the tanks attached end-to-end so there is just one tank that is 75 feet deep and it has a radius of 15 feet.

The volume of such a cylinder is the area of the circle times the depth. If you find the area of the circular end (A = pi x radius2 ) and multiply by the depth, you will have the number of cubic feet of water that the tank holds.

Then if you multiply by the conversion factor 7.5 gallons / cubic foot, you get the number of gallons of water.

Hope this helps,
Stephen La Rocque.

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