Did you know? A 20% discount followed by another 10% discount is equivalent to a 28% reduction, not 30%. And a GST-inclusive displayed price should not have GST added to it again. These examples show how one mistaken percentage base can change an entire answer.
Quick answer: decide what amount represents the original 100%; distinguish GST-inclusive prices from values before GST; use multiplying factors for successive changes; and compare shopping packages by the same unit quantity, such as dollars per 100 g or per litre.
The 2027 G3 Mathematics K310 syllabus includes percentage comparisons, reverse percentages and household finance contexts including taxation. As of 25 October 2026, the IRAS current GST rate is 9% for standard-rated supplies. The fictional shopping figures below are learning examples, not advertisements or advice on a specific business.
Three essential checks
- Find the original percentage base before applying a change.
- Read whether the price already includes GST; do not add a tax twice.
- Compare best-buy offers using a common quantity and the actual final prices.
Jump to a worked question
- The question every percentage problem must answer
- Current GST rate and who can charge it
- Worked example 1: add GST to a clearly GST-exclusive amount
- What consumers should understand about displayed prices
- Worked example 2: reverse GST from a tax-inclusive total
- Another reverse percentage: find the price before a discount
- Two discounts do not usually add
- Discount then GST: an invented calculation with explicit assumptions
- Compare different pack sizes using the same unit
- Best-buy choice after a discount
- An offer can look cheaper but cost more overall
- Percentage points versus percentage growth
- A bill with a fixed fee and a discounted usage charge
- Estimate the answer before the calculator
- A seven-day practice plan using invented shopping figures
- Frequently asked questions about GST and best buys
The question every percentage problem must answer
What is 100%? If an item costs S$80 before a 20% discount, then S$80 is the original 100% and the new price is 80% of that, or S$64.
If the only number supplied is the reduced price S$64, the original S$80 must be recovered by dividing by 0.8, not by adding 20% of S$64. That would calculate S$76.80 and use the wrong percentage base.
Write a label under every amount: before change, after change, GST-inclusive, GST-exclusive, or per unit. Those labels prevent more mistakes than another calculator shortcut.
Current GST rate and who can charge it
Singapore’s Inland Revenue Authority (IRAS) currently states a GST rate of 9%. GST-registered businesses generally charge that rate for standard-rated supplies. Some transactions are zero-rated or exempt, so it is incorrect to claim that every purchase automatically attracts 9% GST.
IRAS also explains that not all businesses are GST-registered; only GST-registered businesses may charge GST under the applicable rules. In an exam-style calculation, follow the conditions expressly stated in the problem.
For this article, we use explicitly hypothetical standard-rated transactions. National tax rules can change; the dated current-rate statement is background for these examples, not a guarantee of future rates.
Worked example 1: add GST to a clearly GST-exclusive amount
Hypothetical invoice calculation: An expressly stated pre-GST taxable value is S$80, and 9% GST applies. GST is 0.09 × 80 = S$7.20.
The GST-inclusive total is S$80 + S$7.20 = S$87.20. Equivalently, multiply the pre-GST amount by 1.09.
The key is that the example deliberately supplies a pre-GST taxable value for a calculation. It is not claiming retailers may display a GST-exclusive price by itself as the final consumer price.
What consumers should understand about displayed prices
IRAS consumer guidance explains that GST-registered businesses generally must display final, GST-inclusive prices to the public, with specific exceptions such as qualifying hotel and food-and-beverage establishments that impose service charges and disclose the applicable additions.
That means a consumer should not automatically add 9% again to an ordinary displayed GST-inclusive price. Doing so double-counts a tax component that is already part of the amount.
For a mathematical word problem, check the exact wording: ‘before GST’ and ‘including GST’ are not interchangeable. This reading skill matters even when the multiplication itself is easy.
Worked example 2: reverse GST from a tax-inclusive total
Suppose a fictional standard-rated item’s GST-inclusive total is S$109 at a 9% rate. The pre-GST amount is S$109 ÷ 1.09 = S$100.
The GST component is S$109 − S$100 = S$9. A mistaken operation such as subtracting 9% of S$109 would give S$99.19, not the original S$100.
Why? The 9% rate is calculated against the pre-GST base. The final S$109 represents 109% of the pre-GST amount, not 100%.
Another reverse percentage: find the price before a discount
A fictional school bag costs S$72 after a 20% discount. Since the sale price represents 80% of the original, write 0.8 × original = 72.
The original price is S$72 ÷ 0.8 = S$90. Check: 20% of 90 is S$18; subtract it and you recover S$72.
Do not add S$14.40, which is 20% of the discounted price, unless the problem specifically requests a calculation based on that new base.
Two discounts do not usually add
A fictional textbook starts at S$100. It receives a 20% reduction, leaving S$80. A further 10% reduction is then applied to that reduced price, leaving S$72.
The combined decrease is S$28 from the original S$100, equivalent to 28%. Multipliers give the same result: 0.8 × 0.9 = 0.72.
Adding 20% + 10% = 30% would be incorrect because the second percentage is based on a lower value. When percentages apply sequentially, their bases matter.
Discount then GST: an invented calculation with explicit assumptions
Teaching exercise only: The original amount is S$100 before GST. The taxable value is discounted by 20%, leaving S$80, and 9% GST is applied to that discounted taxable amount.
The final result is S$80 × 1.09 = S$87.20. This is a mathematical model under stated assumptions, not an advertisement or an assertion about a particular retailer’s promotional terms.
If a displayed price were already GST-inclusive, the steps would need to respect that. Never add GST again merely because a promotional discount is mentioned.
Compare different pack sizes using the same unit
Fictional GST-inclusive shelf prices: Pack A contains 750 g at S$6.60; Pack B contains 1.2 kg at S$9.60. Convert B’s quantity to 1200 g so both are comparable.
Pack A costs 6.60/750 × 100 = S$0.88 per 100 g. Pack B costs 9.60/1200 × 100 = S$0.80 per 100 g.
At those stated prices, Pack B has the lower unit price. This only evaluates price per mass; a real purchase may also depend on storage, expiry, waste or whether the buyer actually needs the larger quantity.
Best-buy choice after a discount
Suppose a different fictional 600 g pack costs S$5.40 and is then reduced by 10%. The new price is 5.40 × 0.9 = S$4.86.
Its unit price is 4.86/600 × 100 = S$0.81 per 100 g. Compare against a 900 g pack priced at S$7.65: 7.65/900 × 100 = S$0.85 per 100 g.
The smaller discounted pack now has the lower price per 100 g, even though its total pack price and quantity are both lower. Use a common unit after applying the stated promotions.
An offer can look cheaper but cost more overall
Imagine a fictional purchase option with an S$80 deposit and twelve monthly payments of S$38. Total outlay = 80 + 12(38) = S$536.
Against a fictional one-time price of S$480, the instalment option’s total is S$56 more. The relative difference against the S$480 base is 56/480 × 100% ≈ 11.7%.
That calculation does not determine a formal annual borrowing rate and does not evaluate a real financial product. It simply illustrates the syllabus skill of comparing totals under clearly defined conditions.
Percentage points versus percentage growth
If the share of a fictional class choosing an option rises from 40% to 50%, the increase is 10 percentage points.
As a relative percentage change against the original 40%, the increase is (50−40)/40 × 100% = 25%. Both descriptions are correct, but they describe different quantities.
Ask whether the question requests a difference between percentages or a percentage increase from a reference amount. The words determine the denominator.
A bill with a fixed fee and a discounted usage charge
Invented utility example: A household uses 250 units at S$0.28 per unit, with a separate fixed S$6 fee. The usage component is S$70, and the undiscounted total is S$76.
If a 10% discount applies only to the usage component, the saving is S$7. The new total is S$70 − S$7 + S$6 = S$69, before any additional charges not specified.
Applying 10% to the entire S$76 would give a different figure. Always identify exactly which part of an invented bill a rate applies to.
Estimate the answer before the calculator
A 9% tax on S$80 should be a little under S$8, so S$7.20 is plausible. A 20% discount on S$90 should be S$18, so the result near S$72 also makes sense.
If a unit-price calculation suggests that 750 g at S$6.60 costs S$8.80 per 100 g, the result is obviously unreasonable: 100 g is only one-seventh-and-a-half of the pack.
Simple order-of-magnitude checks are an inexpensive defence against keypad errors and decimal-place mistakes.
A seven-day practice plan using invented shopping figures
Day 1: identify the original 100% base. Day 2: practise reverse discounts. Day 3: calculate GST-inclusive and pre-GST values when explicitly specified. Day 4: compare sequential percentage multipliers.
Day 5: compare products by price per 100 g or per litre. Day 6: add fixed fees and instalment totals. Day 7: solve unfamiliar mixed purchasing situations and explain every applied rate.
Classify errors as wrong base, GST counted twice, sequential discounts added, inconsistent pack units, fixed fee misapplied or unreasonable final value. Retest each corrected principle with fresh numbers.
Frequently asked questions about GST and best buys
What is Singapore’s current GST rate? IRAS states 9% as of 25 October 2026. Verify later rates against its live official page.
Do all businesses charge GST? No. GST treatment depends on registration and the nature of the supply.
Should consumers always add GST to a displayed shop price? No. IRAS generally requires GST-registered businesses’ publicly displayed prices to be GST-inclusive, subject to stated exceptions.
How do I recover a pre-GST value from S$109? Under a 9% standard-rated example, divide 109 by 1.09 to obtain S$100.
Are two discounts of 20% and 10% a 30% reduction? No. Sequentially applied, they combine to 28%.
How do I compare different pack sizes? Convert quantities to the same unit, calculate price per unit and account for any stated discounts.
Is this part of G3 Mathematics? Yes. Percentages, reverse percentages and real-world taxation and household-finance contexts are addressed in K310.
Putting the learning into a Bukit Timah small-group lesson
The original eduKateSG Mathematics small-group reference stays unchanged. It describes premium three-pupil tutorials of 1.5 hours weekly, close tutor attention and carefully planned independent practice at 8 Fourth Avenue, Singapore 268674, near Sixth Avenue MRT. This is the stated tuition venue; no additional branch or class vacancy is implied.
A practical lesson starts with each pupil’s own unprompted solution, diagnoses the first wrong mathematical decision and then repairs it through a shorter example. A different, unseen question checks whether the new skill can transfer. Students who arrive with similar marks may need entirely different forms of help.
Ask eduKate Singapore about a Mathematics consultation with a current marked script and the learner’s actual examination year.
2026 O-Level and 2027 SEC are not identical examination routes
The first Singapore–Cambridge SEC examinations are in 2027. SEAB lists G3 Mathematics as K310 and identifies 4052 as the previous O-Level Mathematics subject reference. A Secondary 4 candidate actually sitting in 2026 should use the applicable 4052 documents; do not replace their examination requirements with the 2027 syllabus.
Official syllabus and original teaching reference
- SEAB: official 2027 SEC G3 Mathematics K310 syllabus
- SEAB: 2027 school-candidate G3 subject list
- eduKateSG original Mathematics tutorial programme, unchanged
Continue the four-part learning series
- Enlargement and Reduction: Scale-Factor Clinic
- Box Plots, Median and Interquartile Range
- Probability Tree Diagrams With and Without Replacement
- GST, Reverse Percentages and Best-Buy Comparisons
- Earlier: Statistics and Probability Clinic
- Earlier: Compound Interest and Money Mathematics
Editorial note: The numerical examples are original practice exercises, not leaked examination questions, predictions or guaranteed-score claims. Official requirements take precedence for the candidate’s year.