Area of a circle = πr^2
Circumfrance of a circle = 2πr or πd
If 15 ft is the circumfrance, then
15 ft = 2πr
So, 15/2π = r
Now that r is known, just plug it in/substitute it in to the formula for the circle.
A = (π)(15/2π)(15/2π)
A = 225/4π = 56.25π
Use a calculator.