The curved surface area of a cylinder is $3960\, cm^{2}$. If its radius is $21\, cm$,find its height in $cm$.

$A$ cuboidal box measures $25 \, cm \times 15 \, cm \times 8 \, cm$. Find its total surface area (in $cm^2$).

State whether the following statement is true or false:
The radius of a cone can be $3 \, cm$,slant height $4 \, cm$,and height $5 \, cm$.

Find the curved surface area,total surface area and volume of a hemisphere of diameter $42\, cm$.

$A$ child is playing with building blocks, which are in the shape of cubes, and has built a structure as shown in the figure. If the edge of each cube is $3 \, cm$, find the volume of the structure built by the child. (in $cm^3$)

$A$ cone is $8.4 \, cm$ high and the radius of its base is $2.1 \, cm$. It is melted and recast into a sphere. The radius of the sphere is (in $cm$):