Find 7 + 5.
Incorrect answer: 15
When splitting an addend, use all its parts exactly once. In 8 + 5, adding 2 to make ten leaves 3, not 5.
Using the same addend twice when making ten
Making ten uses part of an addend. Only the leftover part should be added afterward.
The correction
12
Split 5 into 3 and 2: 7 + 3 + 2 = 12.
- Choose the larger addend as your starting point.
- Work out how many more it needs to reach ten.
- Split the other addend into that part and a leftover part; add the leftover to ten.
Try the reasoning in another example
Find 9 + 7.
- Split 7 into 1 and 6.
- 9 + 1 + 6 = 10 + 6.
Answer: 16
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.
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