# A well of diameter 150 cm has a stone parapet around it. If the length of the outer edge of the parapet is 660cm, then the width of the parapet is:(a) 30cm(b) 20cm(c) 25cm(d) 40cm

Verified
146.7k+ views
Hint: The diameter of the well is 150cm, so the diameter of the inner circle is 150cm. So, the inner radius is equal to 75cm, i.e., half of the inner diameter. Also, the length of the outer edge of the parapet is 660cm, i.e., the circumference of the outer circle is 660cm. We know that the circumference of the circle is given by $2\pi \left( radius \right)$ , so use this formula to get the outer radius. Finally find the difference between the inner and outer radius to get the answer.

$\therefore r=75cm$
Also, it is given that the length of the outer edge of the parapet is 660cm, i.e., the circumference of the outer circle is 660cm and we know that the circumference of the circle is given by $2\pi R$ .
\begin{align} & \therefore 2\pi R=660 \\ & \Rightarrow R=\dfrac{660}{2\pi } \\ \end{align}
Now, if we put $\pi =\dfrac{22}{7}$ , we get
$R=\dfrac{660\times 7}{2\times 22}=105cm$
$R-r=105-75=30cm$