That depends what you mean with "a 20 cm. circle". If 20 cm. is the radius, you already have the answer. If 20 cm. is the diameter, the radius is half the diameter. (10cm)
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Radius of a circle= Perimeter of the circle/2*pi Radius is half the diameter Radius is the length of a straight line from the center point of the circle to the edge of the circle.
If it is a 7.1 cm radius circle, then its radius is exactly 7.1 cm!
The diameter of a circle is twice the radius. A circle with a radius of 9 would have a diameter of 18. (9*2=18).
the circumfrence of a circle is pi times it's radius squared.
If a circle has a circumference of 16 cm then the radius is about: 2.546 cm