PSLE Maths Percentage: 5 Mistakes That Cost Easy Marks

Percentage is the most real-world topic in the paper — discounts, GST, interest and pay rises are all things your child already sees every day. That familiarity is exactly why PSLE Maths percentage questions cost easy marks: the questions look simple, so your child rushes them.

Almost every mark lost on this topic comes down to the same five mistakes. Once your child can recognise them, each one takes seconds to avoid.

In this guide, you will find all five mistakes explained in plain language, each with a genuine AGrader P6 question and the full working set out step by step.

Table of Contents:

1. The unitary method: one approach behind almost every percentage question

2. Mistakes 1 and 2: working from the wrong starting amount

3. Mistakes 3 and 4: answering a different question from the one asked

4. Mistake 5: percentage change and choosing the right original

5. The 10-second habit that saves these marks

The Unitary Method: One Approach Behind Almost Every Percentage Question

Before looking at the mistakes, it helps to know the method that solves most of them. Nearly every percentage problem follows the same three-step path.

Step 1 — Start: identify the known percentage and the amount it represents.

Step 2 — Then: find 1% by dividing that amount by the known percentage.

Step 3 — Finish: find what the question asks for by multiplying the 1% amount by the target percentage.

This is the unitary method, and once it becomes automatic, your child's thinking can go into reading the question rather than remembering a formula. It is one of several approaches worth drilling, as we cover in Master PSLE Math: 6 Tips to Solve Common PSLE Math Questions.

One more thing worth memorising is what each situation does to 100%:

Situation	What happens to 100%
A discount	Subtracts — 20% off means your child works with 80%
Interest or an increase	Adds — 4% interest means 104%
GST	Adds 9% — the price after GST is 109%

Keep that table in mind, because four of the five mistakes below come from getting this first step wrong.

Mistakes 1 and 2: Working From the Wrong Starting Amount

The first two errors happen at the very beginning of the question, which is what makes them so costly. If your child starts from the wrong 100%, every line of working afterwards is wrong too.

Mistake 1 — In a 'before' question, the price given is not 100%

When a question gives the discounted price and asks for the usual price, your child must remember that the price paid is not the full price. The price paid is 80% of the original, not 100%.

Question: Jenny paid $288 for a camera. It was on a discount of 20%. What was the usual price of the camera?

Worked solution:

• A 20% discount means $288 is 80% of the usual price.

• 80% → $288; 1% → $288 ÷ 80 = $3.60

• 100% → $3.60 × 100 = $360 (usual price)

Some students take ‘$288 × 1.2’ which gives $345.60 — wrong, because the 20% was taken off the larger original price, not the smaller sale price.

Mistake 2 — Stacking a discount and GST in the right order

The second error is trying to combine two percentages into one. Treat the question as a ladder instead: apply the discount first, then apply GST to the discounted price.

Question: A bag cost $200. Betty bought it at a 20% discount. How much did she pay for the bag after adding 9% GST?

Worked solution:

• 100% → $200

• After 20% discount: 80% → $200 ÷ 100 × 80 = $160

• Add 9% GST: $160 × 109/100 = $174.40

The common mistake is treating ‘20% off then 9% on’ as if it nets to ‘11% off’. It doesn’t — each percentage acts on a different amount.

Mistakes 3 and 4: Answering a Different Question From the One Asked

The next two mistakes have nothing to do with calculation. Your child does the working correctly, then writes down a number the question never asked for.

Mistake 3 — Spending is not the same as the total

Multi-step problem sums tempt your child to solve for 100% and stop there, even when 100% is not the answer.

Question: Lena spent 30% of her savings on some books. She then spent 25% of her savings on a bag. If she had $90 left, how much did she spend altogether?

Worked solution:

• Spent 30% + 25% = 55%, so 45% was left.

• 45% → $90; 1% → $2

• The question asks for spending: 55% → $2 × 55 = $110

Finding 100% (her total savings, $200) is the wrong answer here. Re-reading the final line — ‘how much did she spend?’ — is what saves the mark.

Mistake 4 — 'Both', not 'one'

When a question bundles two items together, your child may find one item and forget to double it, or find the total when only one was asked for.

Question: At a sale, a shop was offering a 20% discount for a shirt. Customers can purchase a second shirt at 50% discount. Jaryl paid $26 for 2 shirts. How much would he have to pay for both shirts if not for the discount?

Worked solution:

• First shirt: 100% − 20% = 80%. Second shirt: 100% − 50% = 50%. Together = 130%.

• 130% → $26; 1% → $0.20

• Both shirts at full price: 200% → $0.20 × 200 = $40

Notice that the two shirts together represent 130%, and the full price of both is 200% — not 100%. Getting this right depends entirely on how carefully the question is read. Careless reading costs marks across the whole paper, as our Walkthrough of PSLE Maths 2024 Questions With Sample Solutions shows.

Mistake 5: Percentage Change and Choosing the Right Original

The last mistake appears in percentage increase and decrease questions, and it catches even confident children. The formula itself is simple; choosing the right number to divide by is the hard part.

Percentage increase or decrease = (change ÷ original) × 100%

Question: There were 2000 visitors at an exhibition on Tuesday. This was 400 fewer than the number of visitors on Monday. What was the percentage decrease in the number of visitors from Monday to Tuesday?

Worked solution:

• Tuesday = 2000, which is 400 fewer than Monday, so Monday (the original) = 2400.

• Percentage decrease = 400 ÷ 2400 × 100% = 16 2/3%

Common Mistake: Dividing by 2000 (the number that happens to be printed) gives 20% — wrong base, wrong answer. ‘From Monday to Tuesday’ means Monday is the starting point.

The 10-Second Habit That Saves These Marks

None of these five mistakes is caused by weak arithmetic. Every one of them is caused by starting to calculate before the question has been fully understood.

Before your child writes the final answer, get them into the habit of asking two questions:

1. 'What is 100% here?' This catches the 'before' questions, the discount-and-GST ladder, and the percentage change trap.

2. 'What exactly am I being asked to find?' This catches the spending-versus-total and the both-versus-one errors.

Four of the five traps above disappear the moment those two questions are answered. In class we drill the unitary ladder until it is automatic, so that your child's thinking goes into reading the question rather than recalling the method.

Percentage rewards care far more than it rewards speed. If your child slows down for ten seconds at the start of each question, the marks that used to slip away will start staying on the page. For the wider revision picture, read 6 Key Strategies for PSLE: Preparing Your Child for P6 Maths.

Parents often tell us the same thing about percentage: their child understands the topic at home but keeps losing marks in the paper. The gap is rarely knowledge. It is the reading, the setting up, and the habit of checking what the question actually wants.

AGrader's Primary Maths Tuition Programme offers weekly Mathematics lessons for Primary 1 to Primary 6 children, with online classes available for Primary 5 and Primary 6. It is aligned with the latest MOE syllabus and supported by in-house weekly worksheets.

Lessons are taught ahead of school using a Step-by-Step Approach, so your child meets each topic in class before it appears in the classroom. The programme teaches a structured heuristics framework of 12 techniques across 4 categories — the same kind of systematic thinking behind the unitary ladder above — alongside revision papers, diagnostic tests and worksheets that mirror the actual exam format. Every enrolled child also receives free, unlimited access to the EverLoop Improvement System, with five modules including Past Year Paper Packs, Basic Build-Up, Revision Packs, Topical Packs and LessonTube, plus bonus resources from three previous levels.

With over 19 locations island-wide, there is likely a centre near you. If PSLE Maths percentage questions are quietly costing your child marks, structured practice is the fastest way to fix it — find out more at https://www.agrader.sg/primary-math-tuition.

👉 Enquire today to secure a slot and get your child started with confidence.

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