Question

8.
The inner diameter of a well is 4.20 metre and its depth is 10 metre. Find the
inner surface area of the well. Find the cost of plastering it from inside at the rate
Rs.52 per sq.m.
The length of a road roller is 2.1m and its diameter is 1.4m. For levelling a ground
500 rotations of the road roller were required. How much area of ground was levelled
by the road roller? Find the cost of levelling at the rate of Rs. 7 per sq. m.
9.​

Answer 1

Given: A well of cylindrical shape.

Inner diameter = 4.20 m

⇒ Inner radius (r) = 4.20/2 = 2.1 m

Height of well (h) = 10 m

To find inner surface area of the wall, we’ll have to apply curved surface area of cylinder:

CSA = 2πrh

Substituting values in the above equation, we get

⇒ CSA = 132 m2

Plastering of the well will be done on the wall only, so we will be using curved surface area of the well for finding cost of plastering the well.

If cost of plastering 1 m2 = Rs. 52

Then cost of plastering 132 m2 = 132 × 52

= Rs. 6864

Thus, inner surface area of the well is 132 m2 and cost of plastering it is Rs. 6864.