Subject: General Aptitude · Chapter: Area · Exam: · Marks: · Difficulty:
A circular swimming pool is surrounded by a concrete wall 4 ft . wide. If the area of the concrete wall surrounding the pool is $\frac{11}{25}$ that of the pool, then the radius of the pool is
(a)8 ft
(b)16 ft
(c)20 ft
(d)30 ft
Answer
Answer (as printed): C
Explanation
Let the radius of the pool be $R \mathrm{ft}$. Radius of the pool including the wall $=(R+4) \mathrm{ft}$. Area of the concrete wall $$\begin{aligned} & =\pi\left[(R+4)^{2}-R^{2}\right] \text { sq. } \mathrm{ft} \\ & =\pi[(R+4+R)(R+4-R] \text { sq. } \mathrm{ft}=8 \pi(\mathrm{R}+2) \text { sq.ft. } \end{aligned}$$ $\Leftrightarrow \quad$ Radius of the pool $=20 \mathrm{ft}$.
Explanation as extracted from the printed page; notation may be imperfect.