Question

A rectangular sheet of paper 40 cm ⨯ 22 cm, is rolled to form a hollow cylinder of height 40 cm. The radius of the cylinder (in cm) is [CBSE 2014]
(a) 3.5
(b) 7
(c) $\frac{80}{7}$
(d) 5

Answer 1

Let the radius of the cylinder be r cm.
Curved surface area of cylinder = Area of rectangular sheet
$$\begin{gathered} \Rightarrow 2\mathrm{\pi }r\left(40\right)=40\times 22 \\ \Rightarrow 2\times \frac{22}{7}\times r\times 40=40\times 22 \\ \Rightarrow r=3.5\mathrm{cm} \end{gathered}$$

Hence, the correct answer is option A.