Find the area of a 7 cm by 3 cm rectangle.
Incorrect answer: 20 cm
A larger perimeter does not always mean a larger area. A 1 by 9 rectangle has perimeter 20 and area 9; a 4 by 4 square has perimeter 16 and area 16.
Confusing area with perimeter
Area measures the inside with square units. Perimeter measures the boundary with length units.
The correction
21 square cm
Area is 7 × 3.
- Decide whether the task asks about a boundary or a covered surface.
- Use one consistent length unit for every side.
- Label perimeter with length units and area with square units.
Try the reasoning in another example
A square has side 5 m. Find its perimeter.
- All four sides are 5 m.
- Add them or multiply 4 × 5.
Answer: 20 m
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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