Question 1057369
It is easiest to start with the water surface and work your way outward.


The surface area of the water is just the area of the circle: {{{pi*d^2/4}}}


Since the diameter is d = 50 ft, the surface area is {{{pi*50^2/4}}} = 1963.5 sq. ft


The 5 ft sidewalk around the perimeter of the pool makes a larger circle, with a diameter of 60 ft.


Find the area of this larger circle, and subtract the area of the water surface that was 
calculated above to get the area of the sidewalk:


Area of sidewalk = {{{pi*60^2/4 - pi*50^2/4}}} = 2827.43 - 1963.50 = 863.93 sq. ft.


Finally, the area of the grass is the difference between the area of the fenced region, minus the area of the larger circle (2827.43 sq. ft):


Area of grass = (80 * 80) - 2827.43 = 3,572.57 sq. ft.