Order of operations: working left to right
A whole-number question with brackets, × and ÷ loses its mark because the sum was done in the order it was written. The arithmetic in each step is right; the order is wrong, so the answer is a clean, confident, wrong number.
What it looks like
Find the value of 60 − 32 ÷ (10 − 6) × 5.
- Brackets first: 10 − 6 = 4
- Then ÷ and × from left to right: 32 ÷ 4 = 8, then 8 × 5 = 40
- Then −: 60 − 40 = 20
Why 35 is wrong: Working left to right: 60 − 32 = 28, 28 ÷ 4 = 7, 7 × 5 = 35.
Why children do it
- Reading goes left to right, and so did every sum in the lower primary years.
- Brackets are seen, but the rule that × and ÷ come before + and − is applied inconsistently.
- Long expressions tempt a child to clear the first two numbers before looking at the whole line.
How to fix it at home
- Have your child underline the brackets, then box every × and ÷, before writing anything.
- Rewrite the expression one operation at a time, carrying the rest of the line down unchanged.
- Ask "what is 2 + 3 × 4?" now and then. If the answer is 20, talk through why it is 14.
Questions parents ask
Does the PSLE allow a calculator for these?
Paper 1 does not. A calculator in Paper 2 follows the correct order automatically, which is why the slip shows up most in Paper 1.
Is BODMAS the right rule?
Yes: brackets, then × and ÷ from left to right, then + and − from left to right. "Of" counts as multiplication.
How does Knowbotic catch it?
The left-to-right answer is a known trap on every order-of-operations question, so a wrong answer is named as this habit and practised until fixed.
Where it shows up
Other careless mistakes
Stop losing marks to the same mistake.
Three PSLE questions, free, no sign-up. You’ll see exactly where the marks went. Then 7 days of everything for S$4.99.