Two circles are drawn below. The diameter of the smaller circle is a radius of the larger circle. What is the ratio of the smaller circle's circumference to the larger circle's circumference?

Let the smaller circle's radius = r ⇒ circumference = 2pi r

And the larger circle has a radius twice the smaller = 2r ⇒ circumerence = 2pi (2r) =

4 pi r

So...the ratio of their circumferences =

[ 2 pi r ] : [ 4 pi r ] = 2 : 4 = 1 : 2