To find the length of the fence to circle the arena, you need to compute for its circumference. The formula for circumference is:
C = π × diameter
C = 3.14 × 20 = 62.8 feet
TIP: For a quick estimate, you can use π = 3. In this problem, the answers given in the choices are quite far apart in value, so estimating will work fine. C = 3 × 20 = 60. The closest answer choice to 60 is 62.80 feet.