The number of significant figures in $0.03240$ is

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

The mass of a box measured by a grocer's balance is $2.30 \ kg$. Two gold pieces of masses $20.15 \ g$ and $20.17 \ g$ are added to the box. What is
$(a)$ the total mass of the box,
$(b)$ the difference in the masses of the pieces to correct significant figures?

$A$ body of mass $m = 3.513 \; kg$ is moving along the $x$-axis with a speed of $5.00 \; ms^{-1}$. The magnitude of its momentum is recorded as (Consider significant figures):

If a calculated value $2.7465 \,g$ contains only three significant figures,the two insignificant digits in it are ............

The mass and volume of a body are $4.237 \ g$ and $2.5 \ cm^3$,respectively. The density of the material of the body in correct significant figures is $:-$ (in $g \ cm^{-3}$)