A running track is the ring formed by two concentric circles. If the circumferences of the two circles differ by 20 pi feet, how wide is the track in feet?

The circumference of the inside circle can be expressed as 2pi R_{1}

The circumfernce of the outside circle can be expressed as 2piR_{2}

So

20pi = 2pi R_{2} - 2pi R_{1}

20pi = 2pi (R2 - R1) divide both sides by 2 pi

10 = R_{2} - R_{1} [ the track is 10 ft wide ]