The surface area of a sphere is:
A=4πr^2
And the circumference of a sphere (or circle) is:
C=2πr
Solving for r, r=C/(2π), and using that for r in the area equation gives you:
A=4π(C/(2π)^2
A=4π(C^2/(4π^2))
A=(4πC^2)/(4π^2)
A=(C^2)/π, since we are told that C=50 ft
A=2500/π ft^2
A≈795.8 ft^2 (to nearest tenth)