Chapter 1 - Section 1.2 - Formulas and Percent - 1.2 Exercises - Page 61: 13

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

Using the properties of equality, in terms of $ r ,$ the given formula, $ 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}