Use 7 + 2 = 9 to find 9 − 7.
Incorrect answer: 16
You can swap the addends, but subtraction is not interchangeable. From 8 − 3 = 5, you cannot conclude 3 − 8 = 5.
Reversing subtraction in a fact family
Addition allows the two parts to swap places. Subtraction begins with the total when finding a missing part.
The correction
2
Removing the part 7 leaves the other part, 2.
- Identify the largest number as the total in a whole-number part–whole family.
- Write an addition sentence in each order.
- Start each subtraction sentence with the total and subtract one part.
Try the reasoning in another example
Use 6 + 4 = 10 to find 10 − 6.
- Ten is the total; six is one part.
- The other part is four.
Answer: 4
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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