It’s a small though important point. The circles on circles explanation provided by Marianne on 16th May, 2014 has an error repeated several times.
Marianne states “The circumference of a circle with radius r is 2 pi r, ∴ if the radius is 4, then the circumference is 8 pi r.”
In fact, the circumference of a circle with a radius of 4 is 8 pi. not 8 pi r. There is no ‘r’ in the answer.