This is the boss. No calculator — today $\pi = 3$, the estimator's round number, and every value is chosen to fit in your head. One shed wall, one long job, and every answer feeds the next: the wall, the gable on top, the openings that stay unpainted, the area that's left, and the dollars it costs. Write each line down as you go — one early slip drips all the way to the till.

Everything from the chapter rides along: a rectangle, a triangle, a circle, and a unit price to close the deal. 5★ under par here is the chapter crown. Shake the tin.