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

Height, `h=22.5m` and diameter, `D=7m`

Radius, `r=7/2cm`

Volume of Earth to be dug out = `pir^2h`

`V=22/7xx7/2xx22.5` = `866.25cm^3`

Now Area of the inner curved surface = `2pirh`

Area = `2xx22/7xx7/2xx22.5` =`495cm^2`

Cost of plastering = `495xx3`

Cost of plastering is Rs. 1485.