Adjust a Mental Sum Around a Friendly Ten
Replace the near-ten addend with the next multiple of ten.
About 15 minutes, plus any extended practice
Supplied materials
- Paper and a pencil; drawing and spoken explanations are welcome. An adult may read directions and options aloud.
- Supplied worked-case record: For 47+29, use 47+30=77 and subtract the extra one: 76.
- All quantities and shape descriptions are supplied in the worked example and questions; no purchase or personal information is needed.
Learn the idea
A nearby multiple of ten can simplify addition. If you temporarily add too much to reach that friendly number, correct the excess afterward. The adjustment must preserve the original sum. This is particularly useful when an addend ends in nine.
Replace the near-ten addend with the next multiple of ten. Add the easy amount, then remove the one extra you introduced. Compare with a direct place-value addition to verify the strategy.
Write the temporary extra amount so you remember to correct it. If you rounded an addend up, correct the sum downward. Write the original addend next to its friendly replacement and mark their difference.
- Write the original addend next to its friendly replacement and mark their difference.
- Follow the worked example. Point to each number or shape feature and say what it tells you.
- Replace the near-ten addend with the next multiple of ten.
- Try the question and then the three practice questions. Check your answers and explain any change you make.
See the method in action
For 47+29, use 47+30=77 and subtract the extra one: 76. Twenty-nine is one less than thirty. The correction matters: leaving 77 would answer 47+30, not the supplied problem. Write the temporary extra amount so you remember to correct it. If you rounded an addend up, correct the sum downward. Write the original addend next to its friendly replacement and mark their difference.
Check your reasoning
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Helpful tips
- Write the temporary extra amount so you remember to correct it.
- If you rounded an addend up, correct the sum downward.
See the answer and explanation
64
Add thirty, then remove the extra one. The result is 64.
Take it further
Find 68+19. Expected: 68+20−1=87.
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More practice: try new cases
Choose an answer for each problem. Explain your method on paper, then use the feedback to decide what to revisit.
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Check the subject sources
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