The wheel of a motorcycle has a radius of $35 \, cm$. How many revolutions per minute must the wheel make to maintain a speed of $66 \, km/h$?

The circumference of the wheels of a truck is $440 \, cm$. They make $250$ rotations per minute. Then,the speed of the truck is $\ldots \ldots \ldots \ldots \, km/h$.

In the given diagram,$\Delta ABC$ is a right-angled triangle in which $m \angle B = 90^{\circ}$ and $AB = BC = 14 \text{ cm}$. $A$ minor sector $BAPC$ is a sector of a circle with center $B$ and radius $BA$. $A$ semicircle arc $\widehat{AQC}$ is drawn on diameter $\overline{AC}$. Find the area of the shaded region in $\text{cm}^2$.

In a circle with radius $14 \, cm$,$\overline{OA}$ and $\overline{OB}$ are radii perpendicular to each other. Then,the area of the minor sector corresponding to $\angle AOB$ is $\ldots \ldots \ldots \, cm^2$.

If the radius of a circle is doubled,its area becomes $\ldots \ldots \ldots$ times the area of the original circle.

As shown in the diagram,$\overline{ OA }$ and $\overline{ OB }$ are two radii of $\odot( O , 21 \text{ cm} )$ perpendicular to each other. If $OD = 10 \text{ cm}$,find the area of the shaded region. (in $\text{cm}^2$)