Area of a circle is PI x r^2, where r is the radius ( half the diameter).
Set up the equation to solve for r:
40 = PI x r^2
r^2 = 40/PI
r = SQRT(40/PI)
r = sqrt(40/3.14)
r = sqrt(12.74)
r = 3.57
Diameter = 2 x 3.57
Diameter = 7.14 feet.
