A dartboard has a diameter of 18 inches. What are its radius and circumference?

Diameter of the dartboard as given in the question = 18 inches

Then

Radius of the dartboard will be = 18/2 inches

= 9 inches

We already know that

Circumference of a circle = 2π r

We also know that the value of π = 3.14159

Then

Circumference of the dartboard = 2 * 3.14159 * 9 inches

= 56.548 inches

= 56.55 inches

So the radius of the dartboard is 9 inches and the circumference of the dartboard is 56.55 inches. I hope the procedure is absolutely clear to you.