The formula for the circumference is:
[tex]2\pi r[/tex]
So to find first radius:
[tex]2\pi r = 12\pi \\ \\ \pi r = 6\pi \\ \\ r = 6[/tex]
Second radius:
[tex]2\pi r = 18\pi \\ \\ \pi r = 9\pi \\ \\ r = 9[/tex]
The ratio between them is:
[tex] \frac{6}{9} = \frac{2}{3} = 0.66667[/tex]
