Explanation:
FIVE rotations is FIVE circumferences
so EACH circumference is 40.56 ft. / 5
circumference is pi * diameter
which re-arranges to circumference / pi = diameter
Using 40.56/5 as circumference this becomes
40.56 / (5pi) = diameter = 2.58 ft
multiply this by 12 in/ foot to find inches = 30.98 = ~ 31 inches