A well with 7 m inside diameter is dug 22.5 m deep and the earth taken out of it has been spread around it to a width of 10.5 m to form an embankment. Find the height of the embankment so formed .

The well is in the form of a cylinder with radius = 3.5 m and depth = 22.5 m. 
Then, the earth dug out is spread around to form an embarkment. Volume of earth dug out = Volume of earth on the embarkment.
Volume of earth on the embarkment = Area of embarkment * height.
So, $$\pi\ \times\ 3.5^2\times\ 22.5\ =\ Area\times\ height$$.
Area of embarkment = Area of larger circle - Area of inner circle = $$\pi\ \times\ 14^2-\pi\ \times\ 3.5^2\ =\pi\ \left(14^2-3.5^2\right)$$.
Putting this value of area in the above equation gives us the value of height to be 1.5 m.