This is an example of a series of proportional circles within a sqare.
Source: Uploaded to YouTube by charlie Lindelof on Sep 19, 2011
This is a 30 minute tutorial. A suggestion may be to use these examples as practice with the answer being provided. You might want to scroll through the video instead of watching it in it's entirety.
Source: Published on Mar 19, 2012 by AcademyOfBailey