Appendix A - Review - A.8 Problem Solving: Interest, Mixture, Uniform Motion, Constant Rate Job Applications - A.8 Assess Your Understanding - Page A69: 10

$C=2\pi r,$ where $r$ is the radius of the circle and $C$ is the circumference

Let the circumference be $C$ and the radius be $r$. Circumference $=$product of $\pi$ and twice the radius. The mathematical equation is $C=\pi \times 2r=2\pi{r}$