Chapter 3 - Equations and Problem Solving - Chapter 3 Review Problem Set - Page 137: 26

$r=\dfrac{C}{2\pi}$

Using the properties of equality, in terms of $ r,$ the given equation, $ C=2\pi r ,$ is equivalent to \begin{array}{l}\require{cancel} \dfrac{C}{2\pi}=\dfrac{2\pi r}{2\pi} \\\\ r=\dfrac{C}{2\pi} .\end{array}