Hi Adam,

The area of a circle is $\pi$ times the square of the radius where $\pi$ is approximately 3.1416. In your case, since the diameter is 15 feet the area is

\[\pi \times \left(\frac{15}{2}\right)^2 = 176.71 \mbox{ square feet.} \]

The are of a 4 foot by 8 foot sheet of foam is $4 \times 8 = 32$ square feet.

\[\frac{176.71}{32} = 5.5\]

so you will need at least 6 sheets.

I suggest that you use some graph paper and draw a reasonably accurate diagram to determine the exact number of sheets. A determining factor will be the amount of cutting you wand to do.

I hope this helps,
Penny