Atoms, Moles and Stoichiometry

I learnt that SI (Systeme International) system of units is used as a common system of units for the ease of communication. I've also learnt the SI base units, eg. length (m), mass (kg), the non-SI units used in chemistry, eg. mass (u), pressure (bar or atm), the derived units, eg. energy (J), electric charge (C), and prefixes. There are rules that we have to take note of.
Rules for addition and subtraction:
final value must have only as many decimals as original value.
Rules for multiplication and division:
final value can only have as many significant figures as original value.
5 rules determining the number of significant figures in a number:
1) non-zero digits
2) any zeros btw 2 sig. digits
3) any zeros to the right of decimal point & at the end of number
4) zeros to left of the 1st non-zero digit are not significant
5) zeros in a number without decimal point may or may not be significant.