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

Write 'True' or 'False' and justify your answer:
Two identical solid cubes of side $a$ are joined end to end. Then the total surface area of the resulting cuboid is $12 a^{2}$.

The area of the circular base of a hemisphere is $154 \, cm^{2}$. Then,its total surface area is $\ldots \ldots \ldots \, cm^{2}$.

The radii of a frustum of a cone are $6 \, cm$ and $4 \, cm$. If its height is $9 \, cm$,its volume is $\ldots \ldots \ldots \pi \, cm^3$.

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

Write 'True' or 'False' and justify your answer:
$A$ solid cylinder of radius $r$ and height $h$ is placed over another cylinder of the same height and radius. The total surface area of the shape so formed is $4 \pi r h + 4 \pi r^2$.