The case in three lines. Tara has a £40 envelope and a £38 list of presents. The bookshop offers £10 off when you spend £50, but the small print excludes the jigsaw, which is already reduced, so she would need £41 more of books she had not planned. The offer would cost her £29 over her envelope, and Mr Fenwick asks at the till whether she wants it.

One question to ask. "What would you say to Mr Fenwick?" Then: "Is the offer a bad thing?" It probably isn't, and that is what makes it hard.

One thing to do at the table. Find an offer one of you has seen: "spend £X, get £Y off", "3 for 2", free delivery over a threshold. Write down what you were going to buy anyway, then work out the total with the offer and without it.

The grown-up layer. The case separates a discount on planned spending from extra spending with a smaller price tag, and shows the small print changing what counts. It gives the bookshop owner, Tara's mum and Danya their strongest points. The habits it models are writing the plan before shopping, reading the line under the big letters, and saying what any difference would be taken from.

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