Mental Addition by Compensation
When one addend is close to a “friendly” number, you can make the calculation easier by adjusting it and then correcting the total.
Method
- Choose an addend to adjust.
Pick the number that is easiest to change into a round number such as 10, 20, 100, etc.
- Add or subtract the needed amount.
For example, change 19 to 20 by adding 1, or change 48 to 50 by adding 2.
- Compensate in the other addend or in the final result.
If you increased one addend by 1, subtract 1 from the other part of the calculation or from the sum.
- Compute the easier sum mentally.
Use the round number to make the addition simpler.
Example idea
For a sum like 39 + 18, you can turn 39 into 40 by adding 1. Then calculate 40 + 18 = 58, and compensate by subtracting 1: 57.
Check
Add the numbers in a different way to see if the answer makes sense. You can also estimate: the result should be close to the total of the original numbers, not far away.
Tips
- Round only by a small amount.
- Keep track of the compensation carefully.
- This method is especially helpful when one number is near a multiple of 10 or 100.