50 PSLE Math Common Mistakes Students Make and How to Fix Them

How many times have you seen the same types of common mistakes occurring in your child’s Math papers?

How many times have you looked at your child’s Math paper and spotted the same types of mistakes?

How many marks could your child have saved if they had just avoided those errors?

It can be incredibly frustrating, especially when you know your child understands the concept but still drops marks in the same places. The good news is that most PSLE Math mistakes follow a pattern, and patterns can be learned, recognised, and fixed.

Instead of waiting for your child to make these mistakes and lose precious marks in the exam, why not let them learn from other students’ mistakes first?

That is exactly what we have done at Jimmy Maths. Our tutors have spent years analysing PSLE Math papers across all levels and compiled the most common errors into a free resource your child can study before the exam.

Download our “50 Common Mistakes for Math” for FREE below! Learn to avoid these mistakes to save marks! Click the button below to download.

Learning from Other Students’ Mistakes

Most tuition centres and tutors get students to do corrections and learn from their own mistakes. At Jimmy Maths, we do something different.

We invert the whole teaching process. We show your child the mistakes other students have made and explain exactly why they are wrong before your child ever gets the chance to make them.

This shortens your child’s learning journey significantly. Instead of losing marks first and then understanding the error, your child walks into the exam already knowing the traps to avoid.

All our Math tutors have been in this field long enough to know exactly where students go wrong, and they have helped us put together this list for you.

The KCA Method: The Framework Behind Every Fix

At Jimmy Maths, every student learns to approach questions using three steps:

1. K — Keywords: Spot the key terms in the question, such as “remainder,” “altogether,” “cut into pieces,” or “express as a fraction of.”
2. C — Concepts: Link those keywords to the correct Math concept. For example, “remainder” points to the Branching Method, while “cut into pieces” points to counting cuts, not pieces.
3. A — Apply: Use the right method, show working clearly, and check that the answer makes sense.

This framework stops the most common mistake of all: rushing into a solution before fully understanding what the question is asking.

Whole Numbers Common Mistakes

1. Subtract A from B
Wrong: A – B
Correct: B – A

2. Incorrect Workings
Wrong
30 – 2 × 6
= 12 (Marks will be deducted for wrong workings though the answer is correct)
= 30 – 12
= 18

Correct
30 – 2 × 6
= 30 – 12
= 18

3. Order of Operations
Wrong
50 – 3 × 2
= 47 × 2
= 94

Correct
50 – 3 × 2
= 50 – 6
= 44

4. Values with Different Units
5 kg worth of flour is packed into bags of 100g. How many bags are there?
Wrong
5 ÷ 100 = 0.05
Correct
5 kg = 5000 g
5000 ÷ 100 = 50

5. Cut into Pieces
Tom takes 8 minutes to cut a block of wood into 5 pieces. How long will he take to cut it into 10 pieces?
Wrong
8 ÷ 5 × 10 = 16 min
Correct
5 pieces → 4 cuts, 10 pieces → 9 cuts
8 ÷ 4 × 9 = 18 min

6. Gaps / Intervals
Some pots were placed at equal intervals apart. If the first and the 15^th pot were 42 m apart, how far apart were the first and the 10^th?

Wrong
42 ÷ 15 × 10 = 28
Correct
15 pots → 14 gaps
10 pots → 9 gaps
42 ÷ 14 × 9 = 27

7. Group of Items
3 pens were sold for $20. How much do 18 pens cost?
Wrong
18 × $20 = $360
Correct
18 ÷ 3 = 6
6 × $20 = $120

8. Places
Write 70 tenths as a decimal.
Wrong: 0.70
Correct: 7.0

Fractions Common Mistakes

9. Fraction of
Express $2 as a fraction of 80 cents.
Wrong: ⁸⁰⁄₂₀₀ = ²⁄₅
Correct: ²⁰⁰⁄₈₀ = ⁵⁄₂

10. Fraction with Units
May had ⁸⁄₉ m of ribbon. She used ¹⁄₂ m of it. How much does she have left?
Wrong: ⁸⁄₉ × ¹⁄₂ = ⁴⁄₉
Correct: ⁸⁄₉ – ¹⁄₂ = ⁷⁄₁₈

11. Remainder with Fraction
There was ⁸⁄₉ of a pizza. Each person ate ¹⁄₃ of a pizza. What fraction of a pizza was left?

Wrong
⁸⁄₉ ÷ ¹⁄₃
= ⁸⁄₉ × 3
= ⁸⁄₃
= 2²⁄₃
Remainder = ²⁄₃ (There is a missing step!)
Correct
²⁄₃ × ¹⁄₃ = ²⁄₉

12. Fraction of Total
John spends ¹⁄₃ of his money on A and ¹⁄₅ of his money on B. What fraction of his money does he have left?

Wrong: ⁴⁄₅ × ²⁄₃ = ⁸⁄₁₅
Correct: 1 – ¹⁄₃ – ¹⁄₅ = ⁷⁄₁₅

13. Fraction of Remainder
John spends ¹⁄₃ of his money on A and ¹⁄₅ of the remaining money on B. What fraction of his money does he have left?

Wrong: 1 – ¹⁄₃ – ¹⁄₅ = ⁷⁄₁₅
Correct: ⁴⁄₅ × ²⁄₃ = ⁸⁄₁₅

14. Splitting Fractions
How many ¹⁄₆ are there in 3¹⁄₆?

Wrong: 3 + 1 = 4
Correct: 3 × 6 + 1 = 19

15. Rest of Fraction
²⁄₅ of the apples are red, the rest are green.
Wrong: Red → 2u, Green → 5u
Correct: Red → 2u, Green → 3u, Total → 5u

16. Equal Fractions
¹⁄₃ of A is equal to ²⁄₅ of B.
Wrong
¹⁄₃A = ²⁄₅B
⁵⁄₁₅A = ⁶⁄₁₅B (Do not make denominator the same!)

Correct
¹⁄₃A = ²⁄₅B
 ²⁄₆A = ²⁄₅B (Make numerator the same)

17. Equal Fractions (Multiple)
¹⁄₅ of A is twice of ³⁄₁₀ of B.

Wrong
¹⁄₅A = ³⁄₁₀B
³⁄₁₅A = ³⁄₁₀B

Correct
¹⁄₅A = ³⁄₁₀B
⁶⁄₃₀A = ³⁄₁₀B (Make numerator of A twice of the numerator of B)

18. Compare Fractions of Same Denominator
Arrange ¹⁄₃, ¹⁄₅ and ¹⁄₁₀ in ascending order

Wrong: ¹⁄₃, ¹⁄₅ and ¹⁄₁₀
Correct: The bigger the denominator, the smaller the fraction
¹⁄₁₀, ¹⁄₅ and ¹⁄₃

19. Branching
Josh gave ¹⁄₂ his stickers to his brother and 5 to his sister.
Wrong

Correct
Fractions should add up to 1
Signs should be opposite
Numbers should be the same

To find out the rest of the common mistakes, download our “50 Common Mistakes for Math” for FREE below! Click the button below to download.

Final Thoughts

Every recurring mistake your child makes is feedback, not failure. It is pointing to a gap in how they read the question, which concept they are reaching for, or how carefully they are checking their work. All three of these are skills that improve with the right practice.

At Jimmy Maths, we believe that exposing your child to common mistakes before the exam is one of the most effective ways to save marks.At Jimmy Maths, we believe that exposing your child to common mistakes before the exam is one of the most effective ways to save marks. Combined with the KCA Method, your child will not just know the right answers. They will understand why, and that understanding is what holds up under pressure on exam day.

If your child needs more structured support, explore our PSLE Math online courses and tuition programmes, designed to build both deep understanding and exam confidence.

You and your child have got this. 💪