Our class of 28 is planning a day trip to Glendalough. Here is what it costs: the bus is €280 for the whole class, lunch is €4.50 for each pupil, and entry is €7 for each pupil.

Before we work anything out: which cost is paid once for the whole class, and which are paid for every pupil? Which operation finds the lunch bill for all 28 of us?

The interactive shows the Glendalough budget for our class of 28: bus, lunch and entry. Watch how each line is found. The bus is one price for the whole class. Lunch and entry are a price for each pupil, so we multiply by 28. Then we add the three lines for the total, and share the total between the 28 of us to find what each pupil pays.

Now for a fresh, simpler plan. This time we are pricing a different, cheaper trip — a day at a local farm park, for a smaller group and lower prices, with a budget of €400. Let's build it up cost by cost on the board: first the bus at €280, then lunch at €80, then entry at €40. Watch the running total climb and the remaining amount shrink. Which of these costs stay the same if fewer pupils come, and which shrink?