Here is our end-of-module challenge. We are planning a day-trip to Glendalough for the whole class. We have €800 to spend, there are 28 of us, and the day runs from 9 am to 4 pm.

Before we work anything out: what would we need to figure out to know if this trip actually fits?

We are planning a trip to Glendalough — first the timing, then the money. The interactive builds up the budget one cost at a time against an €800 cap: bus, then tour, then lunch. Watch how tall the bar gets each time, and how much of the cap is still free at the end.

We have €184 left after the bus, tour and lunch. Today we decide together how to spend the rest: some for extras (a snack stop or a souvenir), some kept back as a contingency, and maybe an optional guided walk. A contingency is money kept aside in case a cost goes up or something goes wrong.

We will add each one to the board in turn. First we add €60 for extras. Next we add €50 kept back as contingency. Then we add €40 for the optional walk. Before each new line, we stop and check the remaining amount. The bar climbs but stops well short of €800 — we keep some back on purpose.

Now the harder version: for each plan, spread the budget across every line so the total lands exactly on the cap, with no line left at zero. That forces a real trade-off — spend more on one thing and you must spend less on another.

We work through four plans in order: first the €800 Glendalough trip across bus, tour, lunch and extras; then the tighter €600 Newgrange trip; then a €500 Dublin Zoo trip; and finally the stretch — the same €800 Glendalough trip but for a class of 50, where the fixed bus cost is shared much wider.

Look hard at that last plan. The bus still costs €280 whether 28 or 50 pupils go. Share it out: €280 ÷ 28 = €10 each for a class of 28, but €280 ÷ 50 = €5.60 each for a class of 50. The same bus becomes cheaper per head the more pupils go.