This is the boss. No calculator — the numbers are chosen to fit in your head, the way a good deal should. One jacket, one long problem, and every answer feeds the next: the discount makes the sale price, the tax rides on THAT, and the last line asks what really happened between the sticker and the till. Write each line down as you go — one early slip walks all the way to the register.

The finale hides the chapter's favourite lie: 20% off then 10% tax is NOT 10% off overall, because the two percents act on different bases. Percents of different things never add like dollars — that is the whole reason line three exists. 5★ under par here is the chapter crown. Grab a receipt to scribble on.