here's the solution,
volume of cone :
=》
[tex]v = \dfrac{1}{3} \pi {r}^{2} h[/tex]
=》
[tex]21 = \dfrac{1}{3} \times 3.14 \times4 {}^{2} \times h [/tex]
=》
[tex]h = \dfrac{21 \times 3}{3.14 \times 16} [/tex]
=》
[tex]h = 1.253[/tex]
height = 1.25 ft (approximate)
