Basically its the circumference times 12. Circumference of the circle is 2 * 15 * pi which is 30 pi. 30pi * 12 is 360pi which is around 1130.97 or the last option
Multiply length by width, and height. Than multiply the volume by 7.48 gal. Than you have the amount of water that can fill the whole pool. Than multiply by 0.8. and subtract it from the original volume.
