Question

A well with 10 m inside diameter is dug 8.4 m deep. Earth taken out of it is spread all around it to a width of 7.5 m to form an embankment. Find the height of the embankment.

Answer 1

Let r m be the radius and d m be the depth of the well that is dug.
Volume of the well = πr²d = π(5 m)²(8.4 m) = 660 m³
An embankment has the shape of hollow cylinder with thickness. Its inner radii is equal to the well's radii, i.e. r = 5 m, and its outer radii is R = (5 + 7.5 )= 12.5 cm.
Then, the volume of the embankment = πh(R − r²)
Volume of the well = Volume of the embankment
659.73 m³ = πh((12.5 m)² − (5 m)²)​
Hence, the height of the embankment is 1.6 m.