Find 51 − 26.
Incorrect answer: 35
Do not reverse a column just to subtract the smaller digit from the larger. In 42 − 18, the ones calculation is not 8 − 2; regroup first.
Subtracting the smaller digit from the larger
Subtraction keeps its direction. Exchange a ten for ten ones rather than reversing a difficult column.
The correction
25
Regroup to 4 tens and 11 ones; subtract 2 tens and 6 ones.
- Align place values and compare the ones digits.
- Exchange one ten for ten ones if the top ones digit is too small.
- Subtract ones, then subtract the remaining tens. Check by adding the difference to the subtracted number.
Try the reasoning in another example
Find 60 − 27.
- Rewrite 60 as 5 tens and 10 ones.
- 10 − 7 = 3; 5 − 2 = 3.
Answer: 33
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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