PSLE Maths Percentage: How to Solve Discount and GST Questions

Percentage questions involving discounts and GST can seem straightforward, but they often become challenging when several calculations are combined in one problem. For students preparing for PSLE, understanding the method is more important than simply memorising a formula.

In this guide, we break down key PSLE Maths percentage concepts using clear explanations and practical examples. Students will learn how to calculate discounts, find prices after discounts, work with GST, avoid common mistakes and approach multi-step percentage problems with greater confidence.

What Do Students Need to Know About PSLE Maths Percentage?

Percentage means "out of 100". Before students move on to GST, discounts and more demanding word problems, they should be comfortable moving between percentages, fractions and decimals.

For example:

25% = 25/100 = 1/4 = 0.25

So, 25% of $200 is:

1/4 × $200 = $50

The calculation itself is straightforward. The greater challenge in a word problem is often deciding what quantity the percentage refers to.

Here are several percentage relationships worth knowing:

Recognising these relationships can make calculations quicker, but students should still understand the reasoning behind them.

Why is Finding the Whole so Important in Percentage Questions?

One of the most useful habits in percentage problem solving is asking:

Question: A pair of shoes costs $150. The shop offers a 20% discount. What percentage of the original price does the customer pay?

100% − 20% = 80%

Answer: The customer pays 80% of the original price.

Remember:

• Original price = 100%
• Discount = 20%
• Percentage paid = 100% − 20% = 80%

If a student understands this relationship, there are several ways to solve the problem. Without it, even memorised formulas can be applied to the wrong number.

This becomes especially important when a question contains words such as:

• original price
• discounted price
• amount remaining
• price before GST
• price including GST
• percentage increase
• percentage decrease

Students should pause and establish the base quantity before doing any arithmetic.

How Do You Solve Discount Questions?

A discount reduces the original price.

Question: A school bag originally costs $120 and is offered at a 25% discount.What is the price of the school bag after the discount?

Method 1: Find the Discount First

Discount: 25% × $120 = $30

Price after discount: $120 − $30 = $90

Therefore, the customer pays $90.

Method 2: Find the Percentage Paid

If 25% is removed: 100% − 25% = 75%

Therefore: 75% × $120 = $90

Both approaches are correct.

However, students need to pay attention to what the question actually asks. If it asks for the discount amount, the answer is $30. If it asks for the sale price, the answer is $90.

That small difference in wording matters.

How Do GST Percentage Questions Work?

GST is an additional percentage charged on the price before GST.

Question: A desk costs $300 before GST. GST is charged at 9%.What is the final price including GST?

Solution:

• GST amount: 9% × $300 = $27
• Price including GST: $300 + $27 = $327

Another way of seeing the same calculation is:

• Price before GST = 100%
• Price including 9% GST = 109%

Therefore:

109% × $300 = $327

Quick way to understand it:

Students should use the GST rate provided in the question whenever one is given.

How Do You Solve Questions With Both Discount and GST?

This is where careful reading becomes particularly important.

Question: A bicycle has a marked price of $800. A shop gives a 15% discount, after which 9% GST is charged on the discounted price. How much does the customer pay in total?

Before solving, identify the order:

1. Start with the original price: $800
2. Apply the 15% discount
3. Find the discounted price
4. Calculate 9% GST on the discounted price
5. Add GST to find the final amount

Step 1: Find the Price After Discount

Amount paid after a 15% discount:

100% − 15% = 85%

85% × $800 = $680

Step 2: Calculate GST

GST is calculated on $680:

9% × $680 = $61.20

Step 3: Find the Final Amount

$680 + $61.20 = $741.20

So the final amount is $741.20.

The key point is not simply knowing how to calculate 15% or 9%. Students need to recognise that the base changed from $800 to $680 before GST was calculated.

Why Can't You Simply Subtract the Discount From the GST?

A tempting shortcut would be:

15% discount − 9% GST = 6% discount

This does not correctly describe the situation.

Why?

Because the two percentages are calculated from different amounts.

The discount is calculated from the original $800, while GST in our example is calculated from the discounted $680.

When a question involves more than one percentage change, students should identify the amount each percentage is based on instead of simply adding or subtracting the percentages.

What About Reverse Percentage Questions?

Not every question gives the original amount.

A question might instead give the sale price and ask students to work backwards.

Question: A jacket costs $144 after a 20% discount. What was the original price of the jacket?

What do we know?

• Original price = 100%
• Discount = 20%
• Sale price = 80%
• 80% = $144
• We need to find 100%

After a 20% discount, $144 represents:

100% − 20% = 80%

So:

80% = $144
10% = $18
100% = $180

Final Answer: The original price was $180.

A common mistake would be to find 20% of $144 and add it back. That does not work because $144 represents 80% of the original price, not 100%.

This is exactly why identifying the base quantity matters.

Looking for more ways to support your child's Maths preparation? Read our PSLE Maths: Parents' Guide to Make Kids Maths Monsters

How Should Students Approach Multi-Step Percentage Problems?

Long questions often look difficult because several pieces of information appear together.

Instead of searching immediately for a formula, students can use this routine:

1. Read the full question once without calculating.
2. Underline the quantities and percentages given.
3. Identify the original 100%.
4. Look for words such as "remaining", "after", "before" and "of".
5. Decide whether the base changes during the question.
6. Solve one stage at a time.
7. Write down what each intermediate answer represents.
8. Check whether the final answer is reasonable.

For children who continue to find percentage word problems difficult, recommended PSLE Maths tuition can provide more structured practice and targeted feedback.

Why Percentage-of-a-Percentage Questions Can Be Tricky

Consider this situation:

Question: A child has $500. He spends 20% of his money on a bag. He then spends 25% of the remaining money on books.

Find:

a) How much money is left after buying the bag?

b) How much does he spend on books?

Step 1: Find the amount spent on the bag

20% of $500 = $100

Step 2: Find the remaining money

$500 − $100 = $400

Step 3: Find 25% of the remaining $400

25% × $400 = $100

Answers:

• Money remaining after buying the bag = $400
• Amount spent on books = $100

Remember: The 25% is calculated from $400, not $500, because the question says “remaining money”.

Students who automatically calculate both percentages from $500 would miss this important relationship.

Master PSLE Maths Percentage Through Understanding, Not Shortcuts

Doing well in PSLE Maths questions involving percentage is less about collecting formulas and more about understanding relationships between quantities. Encourage your child to explain each step rather than simply produce an answer. When the reasoning becomes clear, unfamiliar percentage questions become much less intimidating.

At Mavis Tutorial Centre, our Maths lessons help students strengthen key concepts and develop systematic problem-solving habits for school assessments and PSLE preparation.