# How do you solve for r in C=2(pi)r?

$r = \frac{C}{2 \pi}$
This is the formula for Circumference $C$ of a circle given the radius $r$, where $\pi = \frac{22}{7}$ or $3.14$ or $3.14159$ or $3.1416$.
Hence $C \times \frac{1}{2 \pi} = 2 \pi \frac{r}{2 \pi}$ or $\frac{C}{2 \pi} = r$ i.e. $r = \frac{C}{2 \pi}$