Use the least precise decimal place
In 12.5 + 0.003, the raw result is 12.503. Since 12.5 is precise only to tenths, the final answer is 12.5.
Arithmetic rule
Addition and Subtraction Significant Figures
For addition and subtraction, decimal-place precision controls the final answer.
Use the calculatorDo not use the fewest sig figs rule here
Addition and subtraction are about place value. A value with many significant figures can still be less precise by decimal place.
Lab report wording
A clear report sentence is: The result should be reported as 12.5 because addition and subtraction follow the least precise decimal place.