What happens to significant figures when numbers are added or
subtracted? When do I round off?
In science, a standard rounding rule for addition and subtraction has
been adopted to preserve information content in all cases. This is done by keeping all of the digits of a calculation until you are ready to present a final answer.
The best method is to cmplete all of the calculations required and then assess the number of significant figures presented in the problem and round the result to that number of significant figures at the very end of the problem.
Example:
When adding or subtracting numbers, find the number which is known to the fewest decimal places, then round the result to that decimal place.
Example: 21.398 + 405 - 2.9 = 423
(The operand 405 is only known to the ones place, so the result must be rounded to the ones place.)