If the ratio of the areas of two circles is $36:49$, find the ratio between their circumferences.

In the figure below, if the radius of the larger circle is $12 \mathrm{c}\mathrm{m}$ and that of the smaller circle is $8 \mathrm{c}\mathrm{m}$, find the area of the shaded region. $\left(\mathrm{\pi }=3.14\right)$

The diameter of a wheel of a car is $84 \mathrm{c}\mathrm{m}$. Find the number of revolutions it makes to travel $66 \mathrm{k}\mathrm{m}$.

$($Use $\pi =3.14)$

A circle of radius $3 \mathrm{c}\mathrm{m}$ is cut out from a square piece of metal sheet of side $8 \mathrm{cm}$. Find the area of the leftover metal sheet.

$($ Take $\pi =3.14)$