The formula for the circumference of a circle is C = 2πr, so plug the values into the formula. The radius, (r) of a circle is one half of the diameter, so split 21 is half to get a radius of 10.5. Use 3.14 for π.
C = 2πr
C = 2(3.14)(10.5)
C = 6.28 × 10.5
C = 65.94
Rounded to the nearest unit, you get a answer of 66 cm.
