P5 Percentage: Discount and GST, One Clue at a Time

A paper price tag swings in on a string. A magnifying glass glides across it and stops on a blank sticker with a question mark. "Every price tag hides a maths case. Can you crack it?"

That is Discount Detective, one of our Primary 5 Topic Explainers. In 30 seconds it solves two cases: a pair of sneakers on sale, and a board game with GST added. Each case is cracked with two clues, and each ends with a "Case solved" stamp. Watch it first, then read on for the method behind the clues.

Discount Detective: crack the case, one clue at a time.

Discount and GST are part of Primary 5 Percentage in the Singapore syllabus. They are also some of the most useful math your child will ever learn, because they are on every receipt.

The method

Every discount or GST question is the same case with two clues.

Clue 1: Find the percentage part. Look for the percentage on the tag (the "25% off" or the "9% GST") and work out how much money it stands for.

Clue 2: Decide whether to take it away or add it on.

A discount is money taken off the price. Take it away.
GST (Goods and Services Tax) is a tax added to the price. Add it on.

That is the whole method. The detective idea is useful because it slows a child down: find the clue first, then decide what it means, then answer.

Finding a percentage part quickly

Percent means "out of 100". A few friendly percentages make Clue 1 much easier:

50% is one half.
25% is one quarter.
10% is one tenth: move every digit one place to the right.
1% is one hundredth.

Most other percentages can be built from these. For example, 30% is three lots of 10%, and 9% is nine lots of 1%.

Worked examples

Case 1: The sneaker sale (from the video)

Nadia sees $80 sneakers at 25% off. What is the sale price?

Clue 1: Find the discount. 25% is one quarter, so 25% of $80 is 80 ÷ 4 = 20. The discount is $20.

Clue 2: Take it away. It is a discount, so subtract: $80 − $20 = $60.

Case solved: the sale price is $60.

Check another way. If 25% is taken off, Nadia pays the other 75%, which is three quarters. One quarter of $80 is $20, so three quarters is $60. It matches.

Case 2: The board game bill (from the video)

Marcus buys a $50 board game. 9% GST is added. What is the total price?

Clue 1: Find the GST. 1% of $50 is $0.50, so 9% is 9 × $0.50 = $4.50.

Clue 2: Add it on. GST is added, so: $50 + $4.50 = $54.50.

Case solved: the total price is $54.50.

Check another way. With GST, Marcus pays 109% of the price. 100% is $50 and 9% is $4.50, which makes $54.50 again. Note the money is written with two decimal places: $4.50, not $4.5.

Case 3: The jacket sale (try it at home)

A $120 jacket is 15% off. What is the sale price?

Clue 1: Find the discount. Build 15% from friendly pieces. 10% of $120 is $12. 5% is half of that, $6. So 15% is $12 + $6 = $18.

Clue 2: Take it away. $120 − $18 = $102.

Case solved: the sale price is $102.

Check another way. The buyer pays 100% − 15% = 85%. 85% of $120 is $120 − $18, which is $102 again.

Common mistakes

Taking away the percentage, not the money. $80 − 25 = $55 treats the 25 as dollars. Ask, "25 what? Is it 25 dollars, or 25 out of every 100?"
Stopping after Clue 1. Writing $20 as the sale price for the sneakers. The $20 is how much is saved, not what Nadia pays. Ask, "Is that the discount, or the price?"
Adding a discount, or taking away GST. Mixing up the direction of Clue 2. Before working anything out, ask your child to say aloud: "Will the answer be more or less than the price on the tag?"
Slipping a decimal place. 9% of $50 written as $45 or $0.45. A quick sense check helps: 10% of $50 is $5, so 9% must be a little less than $5.
Dropping the zero in money. $54.5 instead of $54.50. Money is always written with two decimal places when it has cents.

Parent tips

Turn receipts into cases. Next time you shop, hand your child the receipt or point at a "20% off" sign and ask, "What would we pay?" Real prices make the method stick.
Ask for the estimate first. "More or less than the tag?" and "About how much?" take five seconds and catch most direction mistakes.
Keep the friendly percentages handy. Practise 50%, 25% and 10% of round numbers until they are quick. Everything else is built from them.
Let the steps be the hint. When your child is stuck, do not give the answer. Ask, "What is Clue 1?" Often that is enough to get going.
Praise the check. When your child works an answer out a second way, say so. Checking is a detective's habit worth keeping.

If your child is also working through other Primary 5 topics, our guide to Primary 5 sample questions explained walks through more examples.

How Singapore Math Drills supports the habit

The end of the video shows the app's own version of clues:

Progressive hints and worked solutions: every question can be opened up step by step, and every question has a full worked solution to read afterwards. "Stuck on a question? Open a hint."
Daily Five drills: five hand-picked drill cards are waiting each day, so a percentage case or two turns up regularly without any planning.
Calm by design: no guilt-tripping and no dark patterns, and your child's progress is only ever measured against their own. A case gets solved at the speed of careful thinking.

Our post on hints that teach, not tell explains why we open a question up one step at a time rather than handing over the answer.

Crack the case, one clue at a time.

Try a sample

Try a free interactive Primary 5 worksheet and see how calm practice feels, with no signup or credit card required.

Try a free Singapore math sample