If the height of a cone is 15 cm and its volume is 770 cm³, find the radius of its base?

$\left(\pi=\frac{22}{7}\right)$

Volume of cone $=\frac{1}{3} \pi r^{2} h$

$\Rightarrow \frac{1}{3} \times \frac{22}{7} \times r^{2} \times 15=770$

$\Rightarrow r^{2}=\frac{770 \times 3 \times 7}{22 \times 15}=49$

$\Rightarrow r=\sqrt{49}=7 \mathrm{~cm} .$