The case in three lines. Tara's laptop dies eleven days before her application essay is due. The shop offers £24.99 a month, which turns out to be £599.76 over two years for a £450 laptop, or an interest-free pay-in-three that needs an adult's name and does not fit her savings plan. A refurbished laptop from the library costs £280, and Tara has to say which one at the till.

One question to ask. "What would you say to Ms Okonjo?" Then: "Was anyone in the shop trying to trick her?" Probably not, and that is what makes it hard.

One thing to do at the table. Find a monthly price on something you have both seen advertised. Work out the total together, then compare it with the price if you paid at once. Say the difference out loud.

The grown-up layer. The case separates the size of one payment from the total cost of borrowing, and an interest-free plan from a free one. It also asks whether a new payment fits a budget that already has jobs. The habits it models are totalling every payment, reading the line under the big number, and noticing what has to stop when something new starts.

The case leaves Tara's answer blank on purpose. Ask for it before you offer yours.