Question 4156
{{{C = pi * d}}} or {{{C = 2*pi*r}}}

Since you need to find th radius, use the second equation, substituting 20 pi for C, the Circumference.  The equation now reads:

{{{ 20*pi = 2*pi* r}}}

Divide both sides by 2 pi, in order to solve for r.

{{{ 20*pi/ 2pi = 2  pi*r/ 2pi}}}

When you reduce the fraction, on the left side the 2 and pi divide out, leaving 10 = r.  Of course this means that the radius is 10 meters.