Project: a discount flyer under a lens

Today you calculate for real: check a flyer

Thirty lessons you learned tools. Today you take them out of the book into the world. Project: take a real discount flyer (paper or online) and check it with a counter’s lens.

You will look for answers to four questions:

Does the stated discount fit? The flyer claims “−30%”, next to it the old and new price. Recalculate whether the percent matches.

How much do you really save? Percents sound grand, korunas are paid. Calculate the saving in korunas.

Which pack size is worth it? The price per unit (per kilo, per litre, per piece) compares a small and a big pack fairly.

Where does the flyer try to confuse? “Up to −50%” (on one thing out of a thousand), a discount from a “recommended price” nobody paid, giant type for the percent and tiny type for the conditions.

This is not a school question with smooth numbers. Real prices are 37.90 — and that is exactly why we train it. Estimate and rounding are allowed weapons today.

Jonáš, Sofie and Filip spread a supermarket flyer on the table. Jonáš recalculates: “Chocolate from 39.90 to 29.90, they write −25%. The discount is 10 korunas… 10 : 39.90 is about 0.25. That checks out.” Sofie compares packs: “Laundry gel 2.5 litres for 189, a litre and a half for 129. Per litre: 75.60 against 86. The big one wins.” Filip waves the third page: “Here is ‘40% off’ crossed from a price they never sold it at. I found it in a price history online.” Sofie writes in a table: two honest pieces, one trick.

The working method of the project

Chuẩn bị (5 minutes): get a flyer — a paper one from the letterbox, or an online flyer of a chain. Pick 6–8 products where you can see the old price, the new price and the claimed discount percent.

A table: on paper make columns: product, old price, new price, claimed discount %, my calculated discount %, saving in Kč, note.

Recalculating the discount: calculated discount = (old − new) : old × 100. Compare with the claimed number. A difference of 1–2% is often rounding; a bigger difference is a find!

Price per unit: with two sizes of the same goods divide the price by the contents (Kč per litre, per kilo). Write which pack won — and by how much.

Hunting tricks. Look for: a discount from a “recommended” price; “up to −X%”; a smaller contents at the old price (900 g instead of a kilo — converting to a unit spots it); “3 for the price of 2” (= 33% off, not 50 — calculate why).

The project conclusion: write three sentences. The most honest offer on the flyer, the cleverest trick on the flyer and one thing you would rethink thanks to calculating.

Recalculating a claimed discount: from 249 Kč to 179 Kč, the flyer claims −30%

Step 1: saving: 249 − 179 = 70 Kč.

Step 2: calculated discount: 70 : 249 = 0.281 → about 28%.

Step 3: comparison: claimed 30, calculated 28. A difference of 2 percentage points — the flyer helped itself by rounding up.

In the table note: “really −28%”. It is not a fraud to sue over, but exactly these small differences should go through your hands. Who calculates is harder to lie to.

Price per unit: 500 g of muesli at 62 Kč vs. 750 g at 99 Kč

Step 1: smaller pack: 62 : 0.5 = 124 Kč a kilo.

Step 2: bigger pack: 99 : 0.75 = 132 Kč a kilo.

Result: the smaller pack is cheaper per kilo — 124 against 132 Kč. A surprise! The “family pack” here is more expensive per unit, which really happens; shops count on nobody recalculating large = good value. The rule from the proportion lessons: always compare via one (the unit), never by the size of the box.

The trick “3 for the price of 2”: what is the real discount?

I take a yoghurt at 15 Kč.

Step 1: three pieces normally: 3 × 15 = 45 Kč. In the deal I pay for two: 30 Kč.

Step 2: saving 15 Kč of 45 Kč: 15 : 45 = 1/3 ≈ 33%.

Result: 33% off, not 50%, as the feeling “one miễn phí” whispers. And a condition: you must buy three. Who wanted one yoghurt and bought three did not save 15 Kč — they spent 15 Kč extra. A discount exists only on a purchase you would have made without it too.

Practice: now you do it 🎲

Now try it for real. You will see all the questions at once, like on paper. Bắt đầu with ten. When it goes well, add more.

A warm-up before the project: the price after a discount. Write only the amount in Kč.

Practise the questions

Project: a flyer under a lens

This is the big exercise for the rest of the lesson. The output is a table and three sentences of a conclusion.

1. Get a flyer and pick 6–8 products with an old and a new price. Bắt đầu a table with columns: product, old, new, claimed discount, calculated discount, saving Kč, note.

2. Recalculate all claimed discounts with the formula (old − new) : old × 100. Write differences in the notes.

3. Find in the flyer one product in two sizes and decide by the price per unit which pack is worth it.

4. Write three sentences of a conclusion: the most honest offer, the cleverest trick, one decision you would make differently after recalculating.

Now you 💪

1. I can check a claimed discount by recalculating from the old and new price.
2. I compare packs by the price per unit, not by the size of the box.
3. I spot at least two flyer tricks and I can explain them with numbers.

What to take from this lesson

• A claimed discount can always be recalculated: (old − new) : old × 100.
• Percents sound, korunas pay — calculate both.
• Price per unit is the only fair comparison of packs.
• “3 for the price of 2” is 33% off — and only on a purchase you wanted anyway.
• Who calculates is much harder to lie to.

© 2026 Ing. Martin Polak / AlgoRhino · Điều khoản sử dụng nội dung