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:
Let, r be the radius
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