Find the amount of water displaced by a solid spherical ball of diameter $0.21 \, m$. (in $, m^3$)

$A$ 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 $cm$).

Twenty-seven solid iron spheres,each of radius $r$ and surface area $S$,are melted to form a single sphere with surface area $S^{\prime}$. Find the radius $r^{\prime}$ of the new sphere. (in $r$)

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 radius of a sphere is increased by $10 \%$. Prove that the volume will be increased by $33.1 \%$ approximately.

How many litres of milk can a hemispherical bowl of diameter $10.5\, cm$ hold (in $, l$)?