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.

In this last budget one category has been given a little too much. Look at what the total does, and work out how far over the cap it goes.

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 one a step trickier than the last. Every category has to get some money, so you cannot leave one on zero, and the total has to land exactly on the cap rather than just near it. We start with the €100 class party, move on to the €200 school trip and then the €150 fundraiser, and finish with the €300 whole-school sports day, which has five categories to fill instead of four.