The circumference of a circle is equal to $72\ \pi$. Find the radius of this circle.<

Given: The circumference of a circle is equal to $72\ \pi$. 

To do: To find the radius of this circle.

Solution:

Let $r$ be the radius of the circle. 

$\therefore$ The circumference of the circle $=2\pi r$

$\Rightarrow 2\pi r=72\pi$

$\Rightarrow r=\frac{72\pi}{2\pi}$

$\Rightarrow r=36\ m$

Thus, the radius is $36\ m$.