What is the number of significant digits for a number that does not represent a measurement?

The order of magnitude of the speed of light in $SI$ units is ........

"Trailing zeros in a number without a decimal point are not significant." Show that this statement is not always true.

The number of significant figures in $0.03240$ is

An object of mass $4.237 \, g$ occupies a volume of $1.72 \, cm^3$. The density of the object to appropriate significant figures is ......... $g \, cm^{-3}$.

Each side of a metallic cube of mass $5.580 \text{ kg}$ is measured to be $9.0 \text{ cm}$. Keeping the significant figures in view,the density of the material of the cube can be best expressed as $X \times 10^3 \text{ kg m}^{-3}$,where the value of $X$ is: