jmc

Suppose the radius of the circle is $r$ cm.

Then the area $M$ is $\pi r^2{\text{ cm}}^2$ and the circumference $N$ is $2\pi r\text{ cm}$.

Thus, $\frac{\pi r^2}{2\pi r}=20$ or $\frac{r}{2}=20$ or $r=\boxed{40}$.