1. **Stating the problem:** Make the letter $r$ the subject of the formula $c = 2\pi r$. 2. **Formula and explanation:** The formula relates the circumference $c$ of a circle to its radius $r$ using the constant $\pi$. 3. **Isolate $r$:** To make $r$ the subject, divide both sides of the equation by $2\pi$: $$r = \frac{c}{2\pi}$$ 4. **Explanation:** Dividing by $2\pi$ reverses the multiplication, isolating $r$ on one side. 5. **Final answer:** $$\boxed{r = \frac{c}{2\pi}}$$