In calculations involving addition and subtraction, the answer's precision is limited by the number with the smallest number of the decimal places.

For example 3.0 m is known to the first decimal place, 6.00 m is known to the second decimal place, the result of addition 3.0 + 6.00 = 9.0 is expressed to the first decimal place. 9.0 has the same number of decimal places as the number with the smallest number of decimal places, which is 3.0 m .