Correctly found x = 21 and calculated angle AOB = 3 × 21 = 63°, but reported angle AOB instead of angle A'OB. These two angles are supplementary, not equal.
PRIMARY 5 · MATHEMATICS
Angles
Top 5 mistakes
Ranked by how often they appear across 82 MOE-aligned practice questions in our archive.
Found the sum 3x + 2x = 5x = 105 and reported this as the answer, rather than using it to solve for x and then computing angle A'OB.
Found angle BOC = 2 × 21 = 42° and computed 180° − 42° = 138°, subtracting the wrong angle from 180° to find angle A'OB.
Assumed all four angles of the kite are equal, like a square, so wrote 78°.
Added the two top angles (78° + 78° = 156°) and used that sum as one bottom angle.
Want to fix these mistakes for your child?
Superholic Lab's quiz engine flags every misconception by name and routes your child to targeted practice in our 3-day Plan Quest. Free 7-day trial.
Start a free 7-day trial →