Question 568172
Area of this circle is 225 pi sq ft.

The area of the circle is pi R^2.  Therefore
225 = R^2
We need to find the square root of 225:
sqrt (225) = 15 ft


This tells us the radius.  BUT, going back to the original problem, the question doesn't ask us for radius, it asks for diameter.  Tricky, tricky, tricky  :-)


The formula relating radius and diameter is:
D = 2R
D = 2(15)
D = 30 ft