The circumference of a circle can be found with this equation:
C = 2
r
"r" is the radius, which is half of the diameter.
The diameter is 20.
20 ÷ 2 = 10
Your radius is 10. Now that you know the radius, you can plug it into the equation:
C = 2
r
C = 2
(10) Multiply 2 and 10 together
C = 20

Your circumference is:
20
inches or 62.8318530718 inches(you should round it if you want lol)