The volume of a cone with radius $6 \, cm$ and height $21 \, cm$ is $\ldots \ldots \ldots \, cm^{3}$.

$A$ cylindrical tank is closed at both the ends by hemispheres. The diameter of the tank is $24 \, cm$ and the total height is $78 \, cm$. How many litres of kerosene can it hold?

The radii and heights of a cylinder and a cone are equal. Then,the volume of the cylinder $= \ldots \ldots \ldots \times$ the volume of the cone.

Total surface area of a hemisphere $= \ldots \ldots \ldots . . .$

If the radius of a sphere is doubled,its volume becomes $\ldots \ldots \ldots \ldots$ times the original volume.

$A$ milk container of height $16 \, cm$ is made of metal sheet in the form of a frustum of a cone with radii of its lower and upper ends as $8 \, cm$ and $20 \, cm$ respectively. Find the cost of milk at the rate of $Rs. \, 22$ per $litre$ which the container can hold. (in $Rs.$)