The circumference of a coin is 18pi cm. What is the​ radius? What is the​ diameter?

Question

The circumference of a coin is 18pi cm. What is the​ radius? What is the​ diameter? I don't understand how to set up the problem, and get the correct answer.

Answer 1

C=2pir

If we know circumference is 18pi then we have

18pi=2pir

Now to solve for radius we see that it is being multiplied by pi so we will do the opposite and divide by 2 pi on both sides of the equal sign to get

9=r

So the radius is 9 and since diameter is 2 times the radius then the diameter is 18.