How to use it
- Round one number to a nearby 10, 100, or 1000.
- Do the easier addition.
- Undo exactly the amount you changed.
See it in action
398 + 247 → 400 + 247 − 2 = 645.
How to verify it
Estimate first: 400 + 250 is about 650, so 645 is sensible.
Common mistake
Compensation must preserve the total; if you add 2 to one number, subtract 2 from the result or the other addend.