Given:

Diameter = 4 m

Height = 21 m

$$V = \Pi r^{2}h$$ $$\Rightarrow \Pi (2)*(2)*(21)$$ $$\Rightarrow V = 84\Pi$$

The volume of soil dug from well = volume of earth used to form the embankment

$$\Rightarrow 84\Pi = \Pi (5^{2} – 2^{2})h$$ $$h = \frac{84}{5 – 4}$$ $$\Rightarrow \frac{84}{21}$$ $$\Rightarrow 4m$$

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