The length of the minute hand of a clock is $7\,cm$. The area of the region swept by it in $20$ minutes is $\ldots \ldots \ldots \,cm^2$.

$AB$ and $CD$ are respectively arcs of two concentric circles of radii $21\, cm$ and $7\, cm$ with centre $O$ (see figure). If $\angle AOB = 30^{\circ}$,find the area of the shaded region.

In $\odot(O, r)$,the length of minor $\widehat{ACB}$ is $\frac{1}{6}$ times the circumference of the circle. Then,the measure of the angle subtended at the centre by minor $\widehat{ACB}$ is ......... (in $^{\circ}$)

In a circle with radius $6.3 \ cm$,an arc subtends an angle of measure $150^{\circ}$ at the centre. Find the length of this arc and the area of the sector formed by this arc.

In $\odot(O, r)$,the length of minor $\widehat{ACB}$ is one-eighth of the circumference of the circle. Then,the measure of the angle subtended at the centre by that arc is $\ldots \ldots \ldots \ldots$ (in $^\circ$)

To warn ships for underwater rocks,a lighthouse spreads a red coloured light over a sector of angle $80^{\circ}$ to a distance of $16.5 \, km$. Find the area of the sea over which the ships are warned. (in $km^2$) (Use $\pi=3.14$)