We will use the circumference formula to find the radius and then multiply it by 2 to find the diameter.
C = 2πr
31.4 = (2)(3.14)(r)
31.4 = 6.28r
5 = r
So our radius is 5 in.
We can multiply it by 2 to get the diameter.
5*2 = 10
So the diameter is 10 inches
