Find the surface area of a sphere with a diameter of $21\, cm$ (in $cm^2$).

Find the curved surface area and the total surface area of a hemisphere with a radius of $14\, cm$.

$A$ hemispherical dome of a building needs to be painted. If the circumference of the base of the dome is $17.6 \, m$,find the cost of painting it,given the cost of painting is $Rs. 5$ per $100 \, cm^2$.

The radius of a spherical balloon increases from $7 \, cm$ to $14 \, cm$ as air is being pumped into it. Find the ratio of surface areas of the balloon in the two cases.

$A$ hemispherical metallic bowl is made of steel,$0.2 \, cm$ thick. The inner radius of the bowl is $4 \, cm$. Find the outer curved surface area of the bowl (in $cm^2$).

Find the surface area of a sphere of radius $10.5 \, cm$.
Assume $\pi = \frac{22}{7}$. (in $, cm^{2}$)