How it works
Ines lives in Lumo, where money is a gold star coin. The kite she wants costs six tiles in Pell, so she needs tiles.
The booth's sign says one tile costs three star coins. That is an exchange rate: how much of one money buys one of another.
A rate works both ways. To turn tiles into stars, multiply by three. To turn stars into tiles, divide by three.
Rates change. A week later one tile cost four stars, so the same kite cost six more stars. Real booths keep a little for their work, and real rates change every day.