The below formula is used to find the circumference of a circle;
[tex]C=2\pi r[/tex]But we know that the diameter of a circle is expressed as;
[tex]d=2r[/tex]Let's replace 2r with d in the 1st equation, we'll then have;
[tex]C=\pi d[/tex]We've been told that the diameter of the circle is 20inches, if we substitute this value into our equation, we'll have;
[tex]C=20\pi[/tex]