Circumference of a circle = 2 × π × r
And Diameter = 2 r
Therefore using the formula!
Circumference of the circle with diameter 20 foot :
2 × π × 10 = 20 π foot
Circumference of the circle with diameter 5 foot :
2 × π × 2.5 = 5 π foot
Difference between the sand paintings' diameter = 20 π - 5 π = 15 π foot