|Themes > Science > Chemistry > Miscellenous > Help file Index > Units > Addition and Subtraction of Significant Figures|
The rule for addition and subtraction for significant figures is simple:
For example, if you add 1.23 and 4.567, you get an answer of 5.797. However,
1.23 and 4.567 have different number of decimal places:
Since there is no third decimal place in 1.23, the answer of 5.80 only has two decimal places and thus three significant figures.
You need to be very careful when adding or subtracting numbers in exponential
notation that you make sure that the decimal place is in the same place on all
the numbers. For example, if you add 1.23*107 and
4.567*106, you should convert one of the numbers so that each has the
same exponent: for example, 1.23*107 is also 12.36. Now
when you add,
Correctly rounded, the answer has only one decimal place and three significant figures.
Example: How many significant figures are there in the answer to the following problem:
Solution: The exponents on the numbers are different, so convert one
of them to be the same as the other. 9.963*10-7 is equal to
0.9963*10-6. Now we can add them:
The answer has three decimal places and four significant figures.