Question

A circular room has a floor surface area of 400 square feet

Answer 1

Consider this explanation:
1) for radius (r):
S=πr², where S - area, r - radius. ⇒ r=√(S/π)=√400=20 ft.;
2) for circumference (L):
L=2πr, where r- radius ⇒ L=2π*20=40π ft.;
3) for the diameter (D):
D=2r, where r - radius, ⇒ D=40 ft..