Given: Dimensions of rectangular sheet: 40cm × 22cm

Height of the Hollow Cylinder : 40cm

Area of the Rectangle is = length × breadth

Curved surface Area of the Cylinder = 2πrh (where r and h are radius and height of cylinder respectively)

Let a₁ be the area of Rectangle

∴ a₁ = 40 × 22 cm²

Let a₂ be the Curved surface area of Cylinder

∴ a₂ = 2 × π × r × 40 cm²

We know that when area of a surface doesn’t change even if its shape is changed.

∴ a₁ = a₂

⇒ 40 × 22 = 2 × π × r × 40

⇒ r = = = = = 3.5 cm

∴ Radius of the Cylinder is 3.5cm