If curved surface area of a cylinder is 1386 sq cm and height is 21 cm, what will be its radius?

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

Curved surface area of cylinder = 2πrh

$\Rightarrow 2 \pi r h=1386$

$\Rightarrow 2 \times \frac{22}{7} \times r \times 21=1386$

$\Rightarrow 44 \times 3 \times r=1386$

$\Rightarrow r=\frac{1386}{44 \times 3}=10.5 \mathrm{~cm} .$