A circular swimming pool is surrounded by a concrete wall 4ft wide. If the area of the concrete wall surrounding the pool is 11/25 that of the pool, then the radius of the pool is?

Correct Answer: 20ft

let the radius of the pool be Rft

 

Radius of the pool including the wall = (R+4)ft

 

Area of the concrete wall = πR+42-R2=πR+4+RR+4-R = 8πR+2 sq feet

 

=> 8πR+2 = 1125πR2 =>11R2 =200R+2

 

Radius of the pool R = 20ft