Addition and Subtraction with Decimal Places

The rule for addition and subtraction is simple, although it makes a lousy English sentence:

Count the number of decimal places in each number in the problem. The answer will have the same number of decimal places as the smallest number of decimal places in the problem. 

For example: 

On your calculator, 20.663 + 32.1 = 52.763 

however the correct answer is 52.8 (this is 52.763 rounded to one decimal place). 

Two more examples: 

30.1 + 0.00334 = 30.1

38 + 4.77 = 43

These rules often produce answers that seem wrong, but they are the correct way to deal with measurements.

Keep in mind that when you need to “trim” a calculator answer down to follow these rules that if the digit after the last number being kept is 0-4 we do NOT change the number. If the digit following the last “kept” number is 5-9 we round up. 

In other words 3.642 rounded to the second decimal place is 3.64, while 3.647 rounded to the second decimal place is 3.65.

Multiplication and Division Problems with Significant Figures

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