Answer
7 mi
Work Step by Step
We know that if point $P$ is a point on a circle of radius $r$, and $P$ moves in a distance $s$ on the circumference of the circle in an amount of time $t$, then the linear velocity, $v$, of $P$ is calculated as $v=\frac{s}{t}$.
As a result, we also know that $s=vt$.
We are given that $v=21$ mi/hr and $t=20 min(=\frac{1}{3} hr)$.
Therefore, $s=(21 mi/hr)(\frac{1}{3} hr)=\frac{21}{3}=7$ mi.