The case in three lines. Four friends book a seaside cottage for £560 over four nights, and one of them can only come for two. Split equally it is £140 each; split by nights it is £160 for a full stay and £80 for the friend who leaves early. Tara's own plan allows £120, a friend has quietly said the same, and Tara has to write the message to the group before anyone pays.

One question to ask. "What should Tara post?" Then: "Is equal always fair?" Let them argue both sides.

One thing to do at the table. Think of something your family shares the cost of: a takeaway, a present, a day out. Ask how it is usually split, who decided, and whether anyone ever minded but did not say.

The grown-up layer. The case is about fair division. Equal shares, shares by use and shares by what people can pay are all real rules, and none is fair everywhere. What usually makes a rule feel fair is that the people paying agreed to it before the money was spent. It gives the friend who wants equal shares her strongest point, that counting nights can feel mean, and gives the friend with a job his, that nobody wants to pay for a bed they never slept in. The habits it models are putting each rule on paper and saying your own limit early, when it is cheapest to say.

The case leaves the line blank on purpose. Ask for it before you offer yours.