The radii of three metal spheres are $3 \,cm, 4 \,cm$ and $5 \,cm$ respectively. They are melted into a single solid sphere. Find the radius of the new solid sphere (in $cm$).

The front compound wall of a house is decorated by wooden spheres of diameter $21\, cm,$ placed on small supports as shown in the figure. Eight such spheres are used for this purpose,and are to be painted silver. Each support is a cylinder of radius $1.5\, cm$ and height $7\, cm$ and is to be painted black. Find the cost of paint required if silver paint costs $25$ paise per $cm^{2}$ and black paint costs $5$ paise per $cm^{2}.$

The radius and height of a cylinder are $10\,cm$ and $30\,cm$ respectively. Find the cost of painting its outer curved surface at the rate of ₹ $3$ per $cm^2$. (Use $\pi = 3.14$)

$1\, m^{3} = \dots \, cm^{3}$

The volume of a sphere is $4500 \pi \text{ cm}^3$,then its diameter is $\dots \text{ cm}$.

The radius of the base of a cone is $7 \, cm$ and its height is $30 \, cm$. Find the volume of a cone (in $cm^3$).