Answer
Diameter of the Circle: 20 mi
Area of the Circle: 314 mi$^{2}$
Work Step by Step
a)
We know the circumference of the circle. We simple divide the circumference by $\pi$ to get the diameter.
Circumference of a Circle: $C$ = $\pi$$d$
Therefore:
62.8 mi = 3.14$d$
$d$ = 20 mi
b)
Area of a Circle: $A$ = $\pi$$d$$^{2}$ / 4
$A$ = (3.14)(20 mi)$^{2}$ / 4
$A$ = 314 mi$^{2}$
