A coin is a circle. Circumference of a circle is determined using this following formula
2 × π × r = c
with r as the radius, and c as the circumference
Find the radius
To find the radius, use the formula above
2 × π × r = c
2 × π × r = 18π
2π × r = 18π
r = 18π/2π
r = 9
The radius of the circle is 9 cm
Find the diameter
The diameter is twice as long as the radius.
d = 2 × r
d = 2 × 9
d = 18
The diameter is 18 cm