Curved surface area of a hemisphere $= \ldots \ldots \ldots$

State whether the following statement is true or false:
The ratio of the surface areas of two spheres is $81: 49$. Then,the ratio of their volumes is $729: 343$.

$30$ circular plates,each of radius $14\,cm$ and thickness $3\,cm$,are placed one above the other to form a cylindrical solid. Find:
$(i)$ the total surface area
$(ii)$ the volume of the cylinder so formed.

Find the volume of a sphere of radius $2.1 \, cm$. (in $cm^3$)

The radii of two cylinders are in the ratio $3: 7$ and their heights are in the ratio $28: 15$. Calculate the ratio of their curved surface areas.

Find the lateral surface area of a cube with an edge length of $15 \, cm.$ (in $cm^{2}$)