How many cubic meters of earth must be dug out to sink a well 21m deep and 6 m diameter?
Question:

How many cubic meters of earth must be dug out to sink a well $21 \mathrm{~m}$ deep and $6 \mathrm{~m}$ diameter? Find the cost of plastering the inner surface as well at Rs.9.50 per $\mathrm{m}^{2}$.

Solution:

h be the height

here, h = 21m

2r = 6

⟹ r = 6/2

= 3 m

Volume of the cylinder $=r^{2} * h$

= 22/7 * 3 * 3 * 21

= 66 * 9

$=594 \mathrm{~cm}^{3}$

Cost of plastering $=9.5$ per $\mathrm{m}^{3}$

Cost of plastering inner surface = Rs.(594 * 9.50) = Rs. 5643