Question

Suppose the circumference of a circle is 15pi. What is its diameter?

Answer 1

The circumference of a circle is proportional to its diameter.

$C\propto d\\ \\ \therefore \quad C=kd$

Now, when d=1, C=π, therefore:

k=π

This means that:

$C=\pi d$

If C=15π:

15π=πd

Therefore:

d=15