Math CentralQuandaries & Queries


Question from BETH:

i am trying to cover an area for a 15x15 foot round pool with play sand before I set up the pool. My playsand bag is 50 lbs , says it covers .5 cubic feet per bag. How many bags do I need?

Hi Beth,

I am not sure if I understand your question but I think you want to install a pool that measures 15 feet across. You want to put the pool on a base of sand and you want to know the amount of sand you need.

The area of a circular region is $\pi \; r^2$ square feet where $\pi = 3.1416$ and $r$ is the radius of the circle. The diameter of your circle is 15 feet so its diameter is 7.5 feet and thus the area is

\[\pi \; 7.5^2 = 176.7 \mbox{ square feet.}\]

You need to multiply this by the depth of the sand in feet. I don't know what depth you want but I will use 4 inches as an example. 4 inches is $\large \frac{4}{12} \normalsize = 0.33$ feet and thus you need

\[176.7 \times 0.33 = 58.9 \mbox{ cubic feet.}\]

Each bag contains half a cubic foot of sand so you will need $58.9 \times 2 = 118$ bags.

Write back if I have misinterpreted your question,

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