Student divided the remaining area 84 by the length 12 to find a width of 7 cm, then subtracted from 10 to get the square side: 10 - 7 = 3 cm. This incorrectly treats the L-shape as a simple rectangle.
PRIMARY 4 · MATHEMATICS
Area and Perimeter
Top 5 mistakes
Ranked by how often they appear across 28 MOE-aligned practice questions in our archive.
Student took the square root of the remaining area instead of the removed area: sqrt(84) = approx 9.2, rounded to 9 cm.
Student correctly found the removed area = 120 - 84 = 36 cm2, but reported the area value (36) as the side length instead of finding the square root.
42 cm is not correct. Check whether removing the corner notch actually changes the total perimeter — the two removed outer edges are replaced by two equal inner edges.
46 cm is too large. You may have added the notch edges on top of the original perimeter. When a corner notch is cut, the two new inner edges (3 cm and 4 cm) exactly replace the two outer edges that were removed, so the total perimeter stays the same.
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