Math with Significant Figures
Rounding a Number
When rounding off a number to keep only the amount of significant figures you need use the following rules
1 . Addition and Subtraction
2.550 + 3.5001 = 6.0501 (We are not done yet!)
Therefore the answer should have three digits after the decimal. The fourth number is one. Since it's less than five we should round down the final answer to 6.050
52.36095 - 32.232 = 20.12895
Therefore the answer should have three digits after the decimal. The fourth number is nine. Since it's greater than five we should round up the final answer to 20.129
2 . Multiplication and Division
The significant figures in the result, for both multiplication and division, are determined by the least number of significant figures in any number.
An object has a mass of 35.5324 g and a volume of 15.0 cm3, then find the density.
15.0 cm3 has the least number of significant figures (three significant figures), therefore the answer has three significant figures.