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Let the radius of the pool be r. Then area of the wall and pool = $\pi {(r + 4)^2}$ Area of the pool = $\pi {(r)^2}$ Area of the wall = $\pi {(r + 4)^2} - \pi {(r)^2}$ Given $\pi {(r + 4)^2} - \pi {(r)^2}$ = $\frac{{11}}{{25}}(\pi {r^2})$ ${r^2} + 8r + 16 - {r^2} = \displaystyle\frac{{11}}{{25}}{r^2}$ $11{r^2} - 200r - 400 = 0$ Solving r = 20