Question

A well, 21 m deep, is 3 m in radius. Find the cost of cementing the inner curved surface at the
surface area.
rate of 5 per square metre.
​

Answer 1

Given :

• A well, 21 m deep, is 3 m in radius.

To Find :

• Find the cost of cementing the inner curved surface at the surface area.

• Rate of 5 per square metre.

Solution :

We know that a well is in th e form of a cylinder.

• Here, we have to cement the inner curved surface area. So, we will use curved surface area of cylinder formula.

$: \implies \: \: \: \boxed{ \sf \: CSA \: of \: Cylinder =2 \pi rh }$

Substitute all values :

$: \implies \: \: \: \sf \: \: \: \: \: \: \: \underline{ CSA \: of \: Cylinder }=2 \times \frac{22}{ \cancel{7}} \times \cancel{21} \times 3 \\ \\ \\ : \implies \: \: \: \sf \: \: \: \: \: \: \: \underline{ CSA \: of \: Cylinder }=2 \times 22 \times 3 \times 3 \\ \\ \\ : \implies \: \: \: \sf \: \: \: \: \: \: \: \underline{ CSA \: of \: Cylinder }=44 \times 9 \\ \\ \\ : \implies \: \: \: \sf \: \: \: \: \: \: \: \underline{ CSA \: of \: Cylinder }=396m^2$

                         

Hence, the curved surface area is 396 m².

According to the Question :

• The cost of cementing is Rs. 5/m².

$: \implies \: \: \: \sf \: \: \: \: \: \: \: 396 \times 5 \\ \\ \\ : \implies \: \: \: \sf \: \: \: \: \: \: \: 1980$

• Hence, the cost of cementing the inner curved surface at the rate of ₹5per sq.m. is Rs. 1980.