Teaching Explanation

When you add a number that is "almost" a round number, like 19, it's easier to add the round number (20) and then fix it.

Think of it like this: If you want to add 19, add 20 first. But since 20 is one too many, you must subtract 1 at the end to keep it fair. This "compensation" makes the mental math much smoother.

Method Condition

Most effective when one number ends in 7, 8, or 9 (e.g., adding 19, 28, or 37).

Standard Fallback

Standard column addition.

Misconception & Correction

• Misconception: Forgetting to subtract the adjustment at the end, or adding it instead.

Correction: If you rounded up to reach a friendly number, you must subtract the difference. If you rounded down* (less common), you would add.