Find the volume of a sphere with radius $2.1 \, m$ (in $m^{3}$).

$A$ school provides milk to the students daily in cylindrical glasses of diameter $7 \, cm$. If the glass is filled with milk up to a height of $12 \, cm$, find how many litres of milk is needed to serve $1600$ students.

Find the volume of a sphere whose surface area is $1256 \, cm^{2}$. (Use $\pi = 3.14$)

In $\Delta ABC$,$\angle B = 90^{\circ}$,$AB = 6.3 \, cm$,and $BC = 8.4 \, cm$. If the triangle is revolved about the side $AB$,find the volume of the cone so obtained. Similarly,if the triangle $ABC$ is revolved about the side $BC$,find the volume of the cone so obtained.

Find the surface area of a sphere of radius $1.25 \, m$ $(\pi = 3.14)$ (in $m^2$).

$A$ well,$20 \, m$ deep,is $7 \, m$ in radius. Find the cost of cementing the inner curved surface at the rate of ₹ $15$ per $m^2$. (in ₹)