Let the radius of the pool be R fr.

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

Area of the concrete wall = ` pi [(R + 4)^2 - R^2]` sq.ft.

                                         = ` [ pi (R + 4 + R ) (R + 4 - R)] sq.ft =  8 pi (R + 2) sq ft`

`8 pi (R + 2) = 11/25  pi R^2`

`hArr    11R^2 = 200 (R + 2)`

`hArr    11 R^2 - 200 R - 400 = 0`

`hArr   11R^2 - 220R  + 20R - 400 = 0`

`hArr     11 R (R - 20) + 20 (R - 20)` = 0

`hArr    (R - 20 ) ( 11R + 20) = 0`

`hArr     R = 20`

`:.`     Radius of the pool = 20 ft.