The formula that gives the circumference of a circle is:
Circumference = `pi` *2r, where r represents the radius of th circle.
We'll replace the known values within the formula of circumference:
40 = `pi` *2r
r = 40/2`pi`
r = 20/`pi`
We'll take `pi` = 3.14
r = 20/3.14
r = 6.36 cm
The requested radius of the circle is of 6.36 cm.