8 Percentage Mistakes That Cost PSLE Marks (With Fixes)
Percentage is one of the top mark-losers in PSLE Math. Here are the 8 sneakiest percentage traps P5-P6 students fall into — with worked examples showing the fix for each.
8 Percentage Mistakes That Cost PSLE Marks
Percentage appears in almost every PSLE Paper 2 — in discounts, GST, increase/decrease, and “find the original” problems. Most marks aren’t lost because students can’t calculate. They’re lost to the same 8 sneaky traps. Fix these, and you could rescue 10-15 marks.
Percentage is one of the most heavily tested topics in PSLE Mathematics. It connects fractions, decimals, money, and real-life contexts like shopping, GST, and interest — which is why examiners love it.
The frustrating part? Students who know the formulas still lose marks — not because they can’t do the math, but because they fall into the same traps over and over.
Here are the 8 most common percentage mistakes we see P5-P6 students make, with the exact fix for each one.
Mistake 1: Taking the Percentage of the Wrong Number
This is the #1 percentage killer. Students calculate the percentage correctly — but apply it to the wrong value.
The Wrong Base Trap
Question:
A bag costs $80. Its price increased by 25%. What is the new price?
Wrong:
New price = $80 + 25% of $100 = $80 + $25 = $105
Student used $100 (a “round number”) instead of the original price
Why it happens: Students grab a convenient number or get confused about which number the percentage refers to.
Correct:
25% of $80 = \frac{25}{100} \times 80 = \20</p> <p className="text-green-700">New price = \80 + $20 = $100
💡 The Golden Rule
Always ask: “25% of what?” The percentage is always calculated from the original amount — the number you started with before the change happened.
Mistake 2: Confusing ”% of” with ”% more/less than”
These two question types look similar but work completely differently. Mixing them up is an instant mark-killer.
The '% of' vs '% more than' Trap
Question:
Ali has 60 stickers. He has 20% more stickers than Ben. How many stickers does Ben have?
Wrong:
20% of 60 = 12. Ben has 60 − 12 = 48 stickers.
Student took 20% of Ali’s amount, but the “20% more” is compared to BEN
Why it happens: “20% more than Ben” means Ben is the base (100%). Ali is 120% of Ben. Students mistakenly take 20% of Ali’s number instead.
Correct:
Ali = 120% of Ben
120% → 60
1% →
100% → 50 stickers
⚠️ Key Distinction
- “A is 20% of B” → A = 0.2 × B (A is smaller)
- “A is 20% more than B” → A = 1.2 × B (A is bigger)
- “A is 20% less than B” → A = 0.8 × B (A is smaller)
The word “more” or “less” changes everything!
Mistake 3: The Reverse Percentage Blunder
This is the hardest percentage trap and the one PSLE examiners test most often. When a question gives you the final amount and asks for the original, many students work backwards incorrectly.
The Reverse Percentage Trap
Question:
After a 20% discount, a dress costs $64. What was the original price?
Wrong:
20% of $64 = $12.80. Original = $64 + $12.80 = $76.80
Student took 20% of the DISCOUNTED price, not the ORIGINAL price
Why it happens: Students see “20% discount” and immediately calculate 20% of whatever number they have. But the 20% discount was applied to the original price, not $64.
Correct:
Original = 100%. Discount = 20%. So $64 = 80% of original.
80% → $64
1% → \64 \div 80 = $0.80$0.80 \times 100 = **\80**
💡 The Unitary Method Saves Lives
Whenever you see “after a % change, the result is…”, use the unitary method:
- Figure out what percentage the given amount represents
- Find 1%
- Find 100% (the original)
Never take the percentage of the final amount to go backwards!
Mistake 4: Forgetting to Convert Units First
When the part and whole are in different units, students jump straight into the percentage calculation without converting — and get a wildly wrong answer.
The Unit Conversion Trap
Question:
Express 400 ml as a percentage of 2 litres.
Wrong:
Student divided 400 ml by 2 (litres) without converting
Correct:
Convert: 2 litres = 2000 ml
💡 Same Unit Checkpoint
Before dividing, circle both units in the question. If they’re different, convert the bigger unit to the smaller one first:
- km → m (× 1000)
- litres → ml (× 1000)
- kg → g (× 1000)
- m → cm (× 100)
Mistake 5: The “10% Up Then 10% Down = No Change” Myth
This is a subtle trap that even strong students fall for. A percentage increase followed by the same percentage decrease does not bring you back to the original.
The Equal Percentage Trap
Question:
A shirt costs $100. The price increases by 10%, then later decreases by 10%. What is the final price?
Wrong:
$100 — it went up 10% and back down 10%, so it’s the same!
Why it happens: Students think +10% and −10% cancel out. But the 10% decrease is applied to the new, higher price — not the original $100.
Correct:
After 10% increase: \100 \times 1.10 = $110$110 \times 0.90 = **\99**
The final price is $1 less than the original!
⚠️ Why It Doesn't Cancel
The base changes between steps. The 10% increase was based on $100, but the 10% decrease was based on $110. Different bases → different amounts. This is why you must always calculate step by step.
Mistake 6: Applying GST to the Wrong Amount
PSLE loves discount + GST combo questions. The trap: students apply GST to the original price instead of the discounted price (or vice versa).
The GST Order Trap
Question:
A watch costs $200. There is a 10% discount. After the discount, 9% GST is added. What is the final price?
Wrong:
GST = 9% of $200 = $18. Discount = 10% of $200 = $20.
Final = $200 − $20 + $18 = $198
Student calculated GST on the original price, not the discounted price
Correct:
Step 1: Discount first. 10% of $200 = $20. Price = $200 − $20 = $180
Step 2: GST on discounted price. 9% of $180 = $16.20
Step 3: Final = $180 + $16.20 = $196.20
💡 The Step-by-Step Rule
For combo questions (discount + GST), always finish one step completely before starting the next. Read the question carefully to see which comes first. Usually it’s: discount first → then GST on the lower price.
Mistake 7: Not Answering What the Question Asks
Students do all the calculations correctly, then give the wrong answer because they answered a different question from the one that was asked.
The Wrong Answer Trap
Question:
There are 500 students. 60% are girls. How many boys are there?
Wrong:
60% of 500 = 300. Answer: 300
That’s the number of GIRLS. The question asked for BOYS!
Correct:
Girls = 60% of 500 = 300
Boys = 500 − 300 = 200
Or directly: Boys = 40% of 500 = 200
This also happens with increase/decrease questions:
| Question asks for | Students give | Correct answer |
|---|---|---|
| The new price after 20% increase | The increase amount | Original + increase |
| The increase amount | The new price | New price − original |
| The original price | The discounted price | Use unitary method |
| How much more/less | One of the amounts | Difference between two amounts |
💡 The Final Line Check
Before writing your answer, re-read the last line of the question. Underline exactly what it’s asking for. Does your answer match?
Mistake 8: Percentage of a Remainder (Multi-Step Confusion)
When a question involves two percentage changes on the same whole, students often take the second percentage of the original instead of the remainder.
The Remainder Trap
Question:
Mrs Lee had $500. She spent 40% on groceries. Then she spent 25% of the remainder on transport. How much did she spend on transport?
Wrong:
25% of $500 = $125
Student took 25% of the ORIGINAL $500, not the REMAINDER
Why it happens: Students see “25%” and immediately apply it to the only big number they have ($500). They miss the critical word “remainder”.
Correct:
Step 1: Groceries = 40% of $500 = $200
Step 2: Remainder = $500 − $200 = $300
Step 3: Transport = 25% of $300 = $75
⚠️ Watch for These Trigger Words
When you see “remainder”, “the rest”, “what was left”, or “remaining” — STOP. Calculate what’s left first, then take the percentage of that smaller number.
Quick Reference: The 8 Percentage Traps
| # | Mistake | Quick Fix |
|---|---|---|
| 1 | Taking % of the wrong number | Ask: ”% of what?” → always the original |
| 2 | Confusing ”% of” with ”% more/less than" | "more” = add to 100%, “less” = subtract from 100% |
| 3 | Reverse percentage — using final amount as base | Given amount ≠ 100%. Find what % it represents first |
| 4 | Forgetting to convert units | Circle both units. Different? Convert first |
| 5 | Thinking +10% then −10% = no change | Base changes between steps. Calculate each step |
| 6 | GST on wrong amount | Finish discount first, then apply GST to the new price |
| 7 | Not answering what was asked | Re-read the last line before writing your answer |
| 8 | Taking % of original instead of remainder | See “remainder”? Find what’s left first |
How to Stop Making These Mistakes
The good news: once you know these 8 traps, they become easy to spot. Here’s a 3-step habit that prevents almost all of them:
1. Label your percentages
Before calculating, write down what 100% represents. For example:
- “100% = original price = ?”
- “80% = discounted price = $64”
2. Work one step at a time
Never combine percentage steps in your head. Write each step separately:
- Step 1: Find the discount
- Step 2: Find the discounted price
- Step 3: Find the GST
- Step 4: Find the final price
3. Check your answer with common sense
After solving, ask yourself:
- Does a “discount” answer give a lower price? (It should!)
- Does a “20% increase” give a number bigger than the original? (It should!)
- Is the answer reasonable? (A $50 bag with a 10% discount shouldn’t cost $5!)
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