This is the SMALLEST number, not the largest. Re-read the question.
PRIMARY 6 · MATHEMATICS
Algebra
Top 5 mistakes
Ranked by how often they appear across 78 MOE-aligned practice questions in our archive.
This is the MIDDLE number, not the largest. The three numbers are 28, 30, 32.
You set the sum as 3n + 6 = 90, giving n = 28, but then added 6 instead of 4. The largest is n + 4 = 32.
Expanded −3(2b − 1) as −6b − 3 instead of −6b + 3. The student forgot that −3 × (−1) = +3, computing the constant as −3 and getting 5b + 10 − 6b − 3 = −b + 7.
Combined the b-terms incorrectly as 5b − (−6b) = 5b + 6b = 11b, but then wrote b + 13. Alternatively the student treated −6b as +6b when collecting like terms, giving a positive b coefficient.
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