Find the total surface area of a hemisphere of radius $42 \, cm$. (in $cm^2$)

The radius of a cylinder is $10\, cm$ and its curved surface area is $440\, cm^{2}$. Then its height is $\ldots \ldots \ldots$

The length,breadth,and height of a hall are $7\, m$,$5\, m$,and $4\, m$ respectively. It has two doors measuring $3\, m \times 2\, m$ each and four windows measuring $2\, m \times 1\, m$ each. Find the cost of whitewashing all the four walls and the ceiling of the hall at the rate of ₹$200$ per $m^2$.

$A$ conical tent with radius $7\, m$ and slant height $12\, m$ is to be made. How many metres of $2\, m$ wide tarpaulin will be required? If the price of tarpaulin is ₹ $48$ per meter,find the cost of the tarpaulin.

The diameter of a solid hemisphere is $21 \, cm$,then its volume is $\ldots \ldots \ldots \, cm^3$.

$A$ metallic ball of radius $9 \,cm$ is melted and recast into three metallic balls. If the radii of two balls are $1 \,cm$ and $6 \,cm$,find the radius of the third ball (in $cm$).