For a cone,$l = \ldots \ldots \ldots \ldots$

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

$A$ well with radius $2 \, m$ and depth $50 \, m$ is dug and the soil taken out is spread evenly on a circular plot with radius $40 \, m$. Find the increase in the level of the plot in $cm$.

Total surface area of a closed cube with length $4 \, cm$ is $\ldots \ldots \ldots cm^{2}$.

$A$ cylinder is closed at both the ends by $12 \, cm$ high cones. The radius of the cylinder is $5 \, cm$ and the total height of the combined article is $41 \, cm$. Find the total surface area of this article in $cm^2$. $(\pi = 3.14)$

Write 'True' or 'False' and justify your answer:
The curved surface area of a frustum of a cone is $\pi l(r_{1}+r_{2})$,where $l = \sqrt{h^{2}+(r_{1}+r_{2})^{2}}$,$r_{1}$ and $r_{2}$ are the radii of the two ends of the frustum,and $h$ is the vertical height.