M.1—Arithmetic, rounding, percentages, means and significant figures

Apply the correct operation, estimate the scale, convert fractions and percentages, calculate arithmetic means and round the final answer to a precision supported by the data.

For addition/subtraction, decimal places control the final place; for multiplication/division, significant figures control it. Keep guard digits during working and attach units.

The mean of 2.10, 2.20 and 2.30 is 2.20. A 0.250 mol sample in 0.100 dm³ gives 2.50 mol dm⁻³, not 2.5×10⁶.

More digits are not more accuracy, and percentage change is not the same as percentage-point change.