Imagine our class has been given €100 for a class party.

Hands up: what would you spend it on first, and roughly how much would you give it?

Here is a €100 party budget already split four ways. The €100 is our cap — the most we are allowed to spend. Look at the four amounts and the running total (all the amounts added so far), and notice how much is left after each one.

Now look at a €200 school-trip budget split three ways. The amounts are bigger, but the same rule holds: everything must fit under the cap.

This last one is set up to go wrong. Look at the total when one category is a little too big.

Today we build a €120 party budget together. We'll fill four categories in this order: food, then drinks, then decorations, then prizes. After each amount we check how much of the €120 is left. If the last one tips us over, we take money back off one of the others.

Now we work through four budgets, each a step trickier. Every category must get some money. You can't leave one on zero. The total must land exactly on the cap. We do them in this order. First the €100 class party. Then the €200 school trip. Then the €150 fundraiser. Last of all, the €300 whole-school sports day, which has five categories to fill instead of four.