What is the number of significant figures in $3400$?

Time periods of oscillation of the same simple pendulum measured using four different measuring clocks were recorded as $4.62 \,s, 4.632 \,s, 4.6 \,s$ and $4.64 \,s$. The arithmetic mean of these readings in correct significant figures is: (in $\,s$)

The number of significant figures in $4.870 \ m$ is

Calculate the length of the arc of a circle of radius $31.0 \, cm$ which subtends an angle of $\frac{\pi}{6}$ at the centre.

The area of a sheet of length $10.2 \,cm$ and width $6.8 \,cm$ expressed up to the proper number of significant figures is ......... $cm^2$.

The respective number of significant figures for the numbers $23.023$,$0.0003$,and $2.1 \times 10^3$ are: