How many cubic metres of earth must be dug out to sink a well 22.5m deep and of diameter 7m? Also, find the cost of plastering the inner curved surface at Rs. 3 per square metre.

Explanation

Diameter`=7m`
Radius`=7/2m`
Height`=22.5m`
Volume of earth to be dug-out=Volume of the well
`=pir^2h`
`=22/7xx7/2xx7/2xx22.5m^3`
`=866.25m^3`
Area of the inner curved surface
`=2pirh`
`=2xx22/7xx7/2xx22.5`
`=495m^2`
The cost of plastering the inner curved surface`=495xx3=Rs.1485`