Organize a multi-step calculation and check the result using units, inverse operations and sensible rounding.
Identify inputs, units and the quantity to find. Keep the formula separate from substituted values. For a final price after a discount and tax, write price × (1 − discount) × (1 + tax), with rates expressed as decimals.
For 240 reduced by 15%, then increased by 20% tax, the steps are 240 × 0.85 = 204 and 204 × 1.20 = 244.8. Writing the intermediate 204 makes the order and the tax base visible.
Reverse an operation when possible: divide 244.8 by 1.20, then by 0.85, to recover 240. For a loan, verify that the final balance is near zero and that principal repayments total the amount borrowed. A second check should not simply repeat the same mistaken input.
Keep more precision during the calculation than you need to display. Money often uses two decimals, but invoice rules may require each line to be rounded separately. For measured quantities, extra calculator digits do not increase measurement accuracy.
English edition: 16 September 2026.