[tex]l = 2\pi \: r \\ l = 20 \\ 2\pi \: r = 20 \\ r = \frac{10}{\pi} [/tex]
[tex]l = 2\pi \: r \\ l = 5 \\ 2\pi \: r = 5 \\ r = \frac{5}{2\pi} [/tex]
