Find the surface area of a sphere of diameter $3.5\,m$ (in $m^2$).

In a cylinder,if the radius is doubled and the height is halved,the curved surface area will be:

State whether the following statement is true or false:
The lateral surface area of a cube with an edge length of $5 \, cm$ is $150 \, cm^2$.

The total surface area of a closed cylinder is $748\, cm^2$. If its radius is $7\, cm$,find its height (in $cm$).

$A$ cone of height $12 \, cm$ has a volume $616 \, cm^3$. Then its diameter is $\ldots \ldots \ldots \, cm$.

$A$ metal sphere of radius $20\, cm$ is melted and then cast into small hemispheres each of radius $5\, cm$. Find the number of hemispheres thus obtained (in balls).