[tex]circumference = 2 * \pi * radius[/tex]
[tex] \pi = 3.14[/tex]
[tex]Substitute \ 3.14 \ for \ \pi [/tex]
[tex]81.64 cm = 2 * 3.14 * radius [/tex]
[tex]81.64cm / (2 * 3.14) = radius
[/tex]
[tex]81.64cm / 6.28 = radius
[/tex]
[tex]13 cm = radius [/tex]
