Mental Stretch

[bagwa-john]

Find the largest the inner circle can be, defined by the 20 foot chord, which is inside a 20x20 square. That defines the largest the outer circle can be. 100pi is the difference between them.
The smallest the inner circle can be is the center of a circle with a 20 foot diameter, which has a 314 square foot area, or 100pi. All circles in between have the same ratio in their differences of area.