Project: we play shops · Kode bersama Martin

A big game of seller and customer. Prices, paying, change — everything at once.

Open your own shop

Today no new topic. Today we build. You will open a shop at home and everything you have learnt will run for real.

You will need: goods (toys, books, fruit from a bowl), paper slips for price tags, paper or real coins and one customer — a parent, a sibling, even a soft toy a parent speaks for.

In a shop you count all the time: the sum of the shopping, paying, change, a discount for regular customers. Adding to 100, subtracting, money, comparing — everything at once.

Why a shop of all things? Because in a shop maths is not practice, it is a must. If the seller counts wrong, the customer notices. If the customer does not count at all, they may get the wrong change. Playing shops is the most fun test in the world — and nobody marks it.

The project has four steps. Set up the shop, price the goods, sell, and at the end count the takings. The seller counts out loud — that is a rule of the game. Then the customer hears that everything matches.

Ema and Filip opened a living-room shop called Two Number Kids. Ema sells: a soft toy 25 Kč, a toy car 32 Kč, a book 40 Kč. Filip buys the soft toy and the toy car. Ema counts out loud: “25 and 32… tens 50, ones 7, in all 57!” Filip pays with a hundred… actually two 50 Kč coins. Ema gives change by filling up: “57… and 3 is 60… and 40 is 100. I give 43 back.” Filip checks: 57 + 43 = 100. It matches. Then they swap roles and Filip sells. In the evening both report the takings to dad like real shopkeepers — and dad promises he will come shopping tomorrow too.

💡

The seller counts out loud and the customer checks. Then swap roles — buying is counting too.

Building a shop step by step

Step 1: Set up the shop. Pick 6 to 8 things to sell. Spread them on a table or a bench. Make up a name for the shop and write it on a sign.

Step 2: Price the goods. Write a price tag for each thing. Choose prices up to 50 Kč and round or handy: 12, 25, 30, 18… The tag must be visible. (A small tip: prices in fives are the easiest to count.)

Step 3: Sell. The customer picks one or two things. The seller:

Adds the shopping out loud: tens to tens, ones to ones.
Says the total price.
Takes the money and counts the change by filling up.
The customer checks: price + change = what was paid.

Step 4: Count the takings. After five sales, close the shop. Add all the sales on paper — that is the takings. The seller and the customer count it each on their own and compare. The same number = the books match.

An extra for the brave: a 5 Kč discount for a customer who buys two things. Do not forget to subtract the discount!

A customer buys for 18 Kč and 25 Kč. How much do they pay?

I add out loud: 18 + 25.

Tens: 10 + 20 = 30. Ones: 8 + 5 = 13.

I put together: 30 + 13 = 43 Kč.

“Together 43 Kč, please.”

The ones spilled over (13), the new ten is counted in. If I said 33, the customer would correct me — that is why we count out loud.

The shopping costs 43 Kč, the customer pays 50 Kč. Change?

I fill up from the price.

43… and 2 is 45. And 5 is 50.

I give back 2 + 5 = 7 Kč. I hand over a 2 Kč coin and a 5 Kč coin.

Customer check: 43 + 7 = 50. It matches.

If you do not want 2 Kč coins: 43 and 7 is 50 straight away — a 7 Kč coin does not exist, but a 5 Kč and a 2 Kč do.

Takings for the day: 43, 12, 30, 25. How much did the shop make?

I add step by step, in pairs.

43 + 12: tens 50, ones 5, in all 55.

55 + 30: I add whole tens, 85.

85 + 25: 85 + 20 = 105… stop! That is past a hundred. I try again properly: 85 + 5 = 90, 90 + 20 = 110.

The takings are 110 Kč. Past a hundred — the shop did well. Big numbers deserve a double check, so I counted twice.

⚠️

Do not rush the change. First say the price, then take the money, then fill up in peace. A seller who rushes gives the wrong change — and a customer with a check always catches them.

Practice: now you do it 🎲

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

A warm-up before opening the shop: paying and change to 100 Kč.

Practise the questions

Opening the shop

This exercise is played, not written. Paper is for price tags and takings.

Set up the shop: pick goods, write price tags up to 50 Kč, prepare coins and a sign with a name.
Serve five sales as the seller. Count out loud, give change by filling up.
Swap roles: now you buy and check the seller’s counting.
Tutup the shop and count the takings on paper. Compare it with what the customer counted.

Now you 💪

I add a shop of two things out loud and with no mistake.
I give change by filling up and the check matches.
I count the takings for a whole day of play.

✅

Done when: The shop served five sales, the change matched and the takings came out the same for both.

What to take from this lesson

A shop is maths for real: sum, change, takings.
The seller counts out loud, the customer checks.
Change = filling up from the price to what was paid.
Takings counted the same by two people = the books match.
Anyone who can count, nobody in a shop can cheat.