Chapter 10 - Area - 10-6 Circles and Arcs - Practice and Problem-Solving Exercises - Page 654: 24

$20 \pi \ cm$

Let $C$ be the circumference of a circle: $C=2 \pi r$ or, $C=\pi d~~~~(1)$ Here, $r$ represents radius and $d$ is diameter. We are given that $d= 20 \ cm$ Thus, equation (1) becomes: $C=(20)(\pi)=20 \pi \ cm$