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

State the number of significant figures in the following:
$(a)$ $0.007 \;m^{2}$
$(b)$ $2.64 \times 10^{24} \;kg$
$(c)$ $0.2370 \;g \;cm^{-3}$
$(d)$ $6.320 \;J$
$(e)$ $6.032 \;N \;m^{-2}$
$(f)$ $0.0006032 \;m^{2}$

Perform the subtraction considering significant figures: $3.9 \times 10^5 - 2.5 \times 10^4$

Three measurements are made as $18.425 \, cm$,$7.21 \, cm$,and $5.0 \, cm$. The addition should be written as ........ $cm$.

The number of significant figures in $0.03240$ is

$A$ student measuring the diameter of a pencil of circular cross-section with the help of a vernier scale records the following four readings: $5.50\, mm, 5.55\, mm, 5.45\, mm, 5.65\, mm$. The average of these four readings is $5.5375\, mm$ and the standard deviation of the data is $0.07395\, mm$. The average diameter of the pencil should therefore be recorded as: