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

In an experiment on a simple pendulum to determine the acceleration due to gravity,a student measures the length of the thread as $63.2 \,cm$ and the diameter of the pendulum bob as $2.256 \,cm$. The student should take the length of the pendulum to be ........... $cm$.

Taking into account the significant figures,what is the value of $9.99\, m - 0.0099\, m$ (in $, m$)?

The area of a rectangular field (in $m^{2}$) of length $55.3 \ m$ and breadth $25 \ m$ after rounding off the value for correct significant digits is

The volume of a sphere is $1.76 \ cm^3$. The volume of $25$ such spheres,taking into account significant figures,is:

Write and explain the rules for determining significant figures with examples.