You may have thought that the circumference equals $\displaystyle\pi{r}\usepackage{cmbright}\Huge\$ .

The formula for the circumference of a circle is $\displaystyle{C}={2}\pi{r}\usepackage{cmbright}\Huge\$ or $\displaystyle\pi{d}\usepackage{cmbright}\Huge\$ .

In this circle, $\displaystyle{r}={15}\usepackage{cmbright}\Huge\$ .