Hello from MrBillDoesMath!
Answer: 30
Discussion:
The area of a circle of radius R is given by the formula
Pi * R^2
We are given that the area is 225 Pi (Not 255 Pi as stated by the author)
Pi * R^2 = 225 * Pi
Divide both sides by Pi :
R^2 = 225 ---> R = 15
The diameter is twice the radius so the diameter = 30.
Regards, MrB
