Question

A custodian carries a large key ring. He measures it and calculates that it has a circumference of 25.12 centimeters. What is the key ring's radius?

Answer 1

By definition of $\pi$, we have

$\pi=\dfrac{C}{d}$

and we deduce

$d = \dfrac{C}{\pi}$

Now, recall that the diameter is twice the radius:

$2r=\dfrac{C}{\pi}$

and solve for $r$:

$r=\dfrac{C}{2\pi}$

So, divide the circumference by $2\pi$ and you'll find the radius.