Hypothetically asking, let's say there is a circle with an unknown radius. Inside this certain circle, there is a square. The square's four corners touch the circle's circumference. If you remove the square what percentage of the entire circle would be left.
If my explanation was not clear enough, I have made a diagram demonstrating what I am asking: http://s.codepen.io/PartTimeCoder/debug/LRXZPG