Circumference has two formulas: C = pi times diameter and C = 2 times pi times the radius. In this problem you are given the radius. So, you need to fill in the blanks. Step one set up your formula: C= 2 times pi times radius Step two fill in the blanks C= 2 times (what you are using for pi) times 81 The answer is (using 22/7) 509.14 If this is not the answer you were looking for my apologies.
