By the way, it's pi, not pie. just wanted you to know as you may lose marks in tests for spelling (but i may be wrong)
we know circumference is pi multiplied by the diameter. the diameter is 2x the radius we can re-arrange the first fact to get: diameter (or 2x radius) = circumference divided by pi. since the circumference is 24pi we can cancel off pi from the circumference (24pi) and pi. now we have the diameter = 24 since the diameter is 2x the radius you divide both sides by 2 and get: radius=12cm
