Question:
Find the cost of sinking a tubewell 280 m deep, having diameter 3 m at the rate of Rs 3.60 per cubic metre. Find also the cost of cementing its inner curved surface at Rs 2.50 per square metre.
Solution:
Cost of sinking a tube well $=$ Volume of the tube well $\times$ Cost of sinking a tube well per cubic metre
$=\underline{22}_{7} \times\left(1.5^{2}\right) \times(280) \mathrm{m}^{3} \times$ Rs $3.6 / \mathrm{m}^{3}=$ Rs $7128 .$
Cost of cementing $=$ Inner surface area of the tube well $\times$ Cost of cementina per square metre
$=\left(\left(2 \times \frac{22}{7} \times 1.5 \times 280\right) \mathrm{m}^{2}\right) \times$ Rs $2.5 / \mathrm{m}^{2}=$ Rs 6600