Find 36 + 27.
Incorrect answer: 53
Do not forget the regrouped ten when adding the tens column. The small 1 represents ten ones, not one extra one in the answer.
Forgetting the regrouped ten in addition
Ten ones become one ten. That ten must be included when adding the tens column.
The correction
63
13 ones become 1 ten and 3 ones; 3 + 2 + 1 = 6 tens.
- Align ones under ones and tens under tens.
- Add the ones. Trade ten ones for one ten if needed.
- Add the tens, including the regrouped ten, and check with an estimate.
Try the reasoning in another example
Find 48 + 36.
- Ones: 8 + 6 = 14; write 4 ones and regroup 1 ten.
- Tens: 4 + 3 + 1 = 8.
Answer: 84
Explain why each step preserves the quantity or relationship. If you can compute the answer but cannot explain the step, revisit the model before adding more questions.
The short check below reuses the first example as a retrieval question and adds a second question. It checks this method; it does not establish a grade level.
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