The shape is a square, therefore, all sides are equal.
The edge of the circle touches the edge of the square, and thus, one side of the square can be considered to be the same length as the diameter of the circle.
The formula for circumference is :
2"pie"r
Perimeter = 40ft.
Therefore each side = 10ft.
Therefore diameter = 10ft.
Therefore radius = d/2 = 10/2 = 5 ft.
Therefore circumference = 2"pie"5 ft = 10"pie" ft
(don't forget the unit of measurement!)
Hope this helped ~