The strategy
How it works
Round one addend up or down to a nearby ten.
Add the rounded number to the other addend.
Undo the adjustment to keep the total unchanged.
See it in action
Worked examples
Example 139 + 26
- Round 39 up to 40.
- 40 + 26 = 66.
- Take away the extra 1: 66 − 1 = 65.
Answer: 65
Example 248 + 35
- Round 48 up to 50.
- 50 + 35 = 85.
- Take away 2: 85 − 2 = 83.
Answer: 83
A quick heads-up
Common mistakes to avoid
- Rounding one addend and leaving the total unchanged; undo exactly the amount you added.
- Adjusting both addends when only one was rounded.
Your turn
Check your understanding
Choose an answer to see instant feedback and a short explanation. You can take the quiz again any time.
Question 1 of 3
Use compensation: 29 + 14 = ?
29 + 1 = 30; 30 + 14 = 44, then subtract the extra 1 to get 43.
If 58 is rounded up to 60, what adjustment is needed?
You added 2 to the number, so subtract 2 from the final sum.
What is 48 + 17?
Think 50 + 17 = 67, then subtract the 2 added to 48: 65.
Keep the momentum
Try a few practice problems
Choose a comfortable range and work through a short round with the free mental math trainer.