You buy your lunch and it comes to €6.55. You hand over a €10 note. Hands up: what change should the shopkeeper give you back, and how did you work it out in your head?

Watch four change amounts already built on the coin-counter tray. Each one shows the change owed for a different bill and cash paid. Notice how each change is made with the fewest coins and notes — always the biggest coin that fits first.

For the first one, watch how we count on from €6.55: 45c gets us to €7, then €3 to €10 — that's €3.45.

The next one is change from a €20 note. We count on from €13.85: 15c gets us to €14, then €6 to €20 — that's €6.15.

The next one is small change from a €5 note. This change ends in 73c — an amount that doesn't land neatly on a round euro. Hands up: looking at the tray, which small coins make it, and why can't we use fewer? (A few little coins are still needed here, that's expected.)

The last one is change from a €50 note, so we build it with notes first and then the coins.

• €10 for a €5.70 bill
• €5 for a €2.35 bill
• €20 for a €12.15 bill

• €10 paid for €6.55, build the change
• €20 paid for €13.85, build the change
• €5 paid for €3.27, build the change
• €50 paid for €32.40, build the change with the fewest notes and coins