Singapore Mathematics | International Edition: Discount, GST and Interest

Singapore Math guide to discount, GST and interest for families and teachers outside Singapore. What Primary 5 students learn when percentage meets money, how Singapore teaches it, and how to practise it at home.

Percentage becomes real when it changes a price.

A discount takes money off. A tax adds money on. Interest pays money for saving.

In Singapore Mathematics, these three applications sit inside the Primary 5 percentage content, which names “finding discount, GST and annual interest”. Students use the same skill each time: finding a percentage of a quantity, then deciding whether to add or subtract it.

Compound interest and wider household finance come later, in secondary school.

This guide is useful for families and teachers who want to:

• understand what Singapore expects at Primary 5;
• teach money applications of percentage in a clear sequence;
• prepare a child who is moving into a Singapore school; or
• support a student using Singapore Math materials in another country.

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What Discount, GST and Interest Mean in Singapore Mathematics

Each of these is a percentage of an amount of money.

A discount is the amount taken off the usual price. The sale price is the usual price minus the discount.

GST is Goods and Services Tax. It is added to the price of goods and services. Singapore’s GST rate is currently 9%, although questions may state the rate to use.

Interest is money paid for the use of money. A bank pays interest on savings. Annual interest is the interest for one year, given as a percentage of the amount.

The mathematics is the same each time. Find the percentage part, then decide what happens to it.

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Where Discount, GST and Interest Sit in the Singapore Syllabus

These applications belong to the Primary 5 percentage content.

At Primary 5, students learn:

• expressing a part of a whole as a percentage;
• the use of the % symbol;
• finding a percentage part of a whole; and
• finding discount, GST and annual interest.

At Primary 6, students add finding the whole from a part and its percentage, and percentage increase and decrease. These skills let students work backwards, for example from a sale price to a usual price.

Foundation Mathematics includes discount, GST and annual interest in its Primary 6 percentage content.

Level	Money and percentage
Primary 5	Discount, GST and annual interest as a percentage of an amount
Primary 6	Finding the whole from a part, percentage increase and decrease
Secondary 1 to 4	Problems in real-world contexts, including simple and compound interest, taxation, instalments, utilities bills and money exchange

At secondary level, the G3 Mathematics formula list includes compound interest: Total amount = P(1 + r/100)ⁿ. That formula is not part of the primary content.

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How Singapore Teaches Discount, GST and Interest

1. Learn the words first

Usual price, discount, sale price. Price before GST, GST, price after GST. Amount saved, interest rate, interest. Students underline these words before calculating.

2. Find the percentage part

This is Primary 5 percentage work. 20% of $45 is 20/100 × $45 = $9.

3. Decide whether to add or subtract

A discount is subtracted. GST and interest are added. Students say the direction aloud before writing it.

4. Use the 100% model

The usual price is 100%. After a 20% discount, the customer pays 100% − 20% = 80% of the usual price. This gives a quick check: 80% of $45 is $36.

5. Watch which amount the percentage is taken of

A discount is a percentage of the usual price. If GST is charged after a discount, it is a percentage of the sale price, not the usual price.

6. Check with estimation

A 9% tax should add a little less than 10%. A discount should always leave a price lower than the usual price.

Children who rush to multiply often add when they should subtract. Naming the direction first prevents most errors.

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Worked Examples

In this question, GST is charged on the price after the discount. Always read which amount each percentage belongs to.

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Common Mistakes

• Answering with the discount instead of the sale price. The question asks what the customer pays, and the child writes $20. Fix: underline the final question and check that the answer matches it.
• Taking GST on the wrong amount. In the jacket question, a child finds 9% of $80. Fix: mark which price each percentage belongs to before calculating.
• Combining percentages. A child says “25% off, then 9% on, is 16% off”. That gives 84% of $80, which is $67.20, not $65.40. Fix: work in steps, because the second percentage is taken of a different amount.
• Finding the discount percentage from the sale price. A $40 bag sells for $34. The discount is $6, and the child writes 6/34. Fix: a discount percentage is out of the usual price, so it is 6/40 = 15%.
• Writing money carelessly. A child writes $5.4 for five dollars forty cents. Fix: write cents with two digits, as $5.40.
• Treating the rate as dollars. “2% per year” becomes $2 of interest. Fix: find 2% of the amount saved.

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Teaching Discount, GST and Interest Outside Singapore

The methods transfer to any country. A few points are worth knowing.

• Local taxes. GST is the Singapore name. Your country may have a different tax with a different name, rate and rules. Teach the method with whatever rate a question states.
• Rates change. Singapore’s GST rate is currently 9%. Use the rate a question gives, rather than a rate remembered from elsewhere.
• Currency. Singapore questions use Singapore dollars and cents. Any currency with 100 smaller units works the same way.
• Timing. In Singapore’s current syllabus, these applications are Primary 5 work. Decide by readiness rather than age. The Singapore Math Grade Levels guide explains why direct conversions are unreliable.
• Calculators. PSLE Paper 2 allows calculators but Paper 1 does not, so students need mental methods for 10%, 5% and 1% of an amount.

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How to Practise Discount, GST and Interest at Home

A short, regular routine works better than one long weekend session.

1. Pick three prices from a shop or catalogue and find 10% of each mentally.
2. Apply a stated discount to one price and say whether you added or subtracted.
3. Add a stated tax to a different price, and estimate first.
4. Work out one year’s interest on a savings amount at a simple rate such as 2% or 3%.
5. Solve one two-step question, such as a discount followed by a tax, and check each step.
6. Once a week, reverse a question: given the price and the discount, find the discount percentage.

Ten to fifteen minutes is enough when the work is focused.

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Try These

1. A pair of shoes costs $60. There is a 15% discount. What is the sale price?
2. A meal costs $40 before GST. GST of 9% is charged. What is the total cost?
3. A bag usually costs $40 and is sold for $34. What percentage discount is given?
4. Ben saves $1200 in a bank that pays 3% interest per year. How much interest does he earn in one year?

Answers

1. $51. The discount is 15% of $60 = $9, and $60 − $9 = $51.
2. $43.60. GST is 9% of $40 = $3.60, and $40 + $3.60 = $43.60.
3. 15%. The discount is $6, and 6/40 = 15/100.
4. $36. 1% of $1200 is $12, so 3% is $36.

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What Comes Next

Money applications grow in two directions.

• In Primary 6 and Secondary 1, Percentage Increase and Decrease compares new and original amounts and works backwards to original prices.
• In secondary school, Personal and Household Finance covers contexts such as simple and compound interest, taxation and instalments.
• In Primary 5, Rate uses money too, as a price per unit.

A student who always names the amount a percentage belongs to is well prepared for all three.

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Frequently Asked Questions

When do Singapore students learn discount, GST and interest?

In Primary 5, as part of percentage. Foundation Mathematics includes the same applications in Primary 6.

What is GST?

Goods and Services Tax, a tax added to the price of goods and services in Singapore. Singapore’s GST rate is currently 9%.

Do primary students learn compound interest?

No. The primary content names annual interest. Compound interest appears in secondary school, and its formula is on the G3 Mathematics formula list.

Is GST charged on the usual price or the sale price?

Read the question. In questions where GST is charged after a discount, it is a percentage of the sale price.

Our country has a different tax. Should we still practise GST questions?

Yes, if your child uses Singapore materials or may join a Singapore school. The mathematics is identical to any percentage tax.

Why does my child keep getting the right number but the wrong answer?

Usually the child stops at the discount or tax instead of the final price. Practise underlining what the question asks for.

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Helpful Reading

• Singapore Mathematics | International Edition: Percentage
• Singapore Mathematics | International Edition: Percentage Increase and Decrease
• Singapore Mathematics | International Edition: Personal and Household Finance
• Primary Mathematics: Ratio, Rate and Percentage | Worked Learning Guide
• Real-World Mathematics: Ratios, Rates, Percentages and Proportions
• Secondary Mathematics: Financial Arithmetic, Growth, Depreciation and Interest
• IRAS: Current GST Rates
• MOE Primary Mathematics Syllabus

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Discount, GST and Interest in Singapore Mathematics

Find the percentage part.

Decide: add or subtract.

Name the amount each percentage belongs to.

Work in steps when the base changes.

Taught this way, money questions stop being traps.

They become ordinary percentage with a direction.

That habit carries a student from Primary 5 into the finance problems of secondary school.

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Ask Bukit Timah Tutor

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If your child is learning Singapore Mathematics at home, in an international school or in a Singapore school, and you are unsure where to begin with this topic, send us a short message with the student’s level and one example of the work.

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Singapore Mathematics | International Edition is a Bukit Timah Tutor guide series for families and teachers outside Singapore. It is not a textbook and is not affiliated with MOE, SEAB or any publisher. Syllabus details were checked against MOE and SEAB documents in October 2026. Always confirm current requirements with your school.

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