What is the square footage of a 20' wide round pool that is 4' deep?

The square footage is the diameter 20' divided by 2 to give a radius of 10, so r=10 pi =3.14 r=10 square footage = pi*R*R to solve 3.14*10*10= 314 square feet. It sounds like you are looking for the volume of this cylinder Volume=4*pi*r*r 1256 cubic feet or 1257.142857142856 cubic feet if you carry the constant pi out more decimal places