This is the boss. No calculator — the values are chosen to fit in your head: sound crosses the room at a round $340\ \mathrm{m/s}$, and the metre pendulum's famous $2.0\ \mathrm{s}$ swing stands in for $g$. One metronome, one long problem, and every answer feeds the next: beats to hertz, hertz flipped to a period, the wavelength each click casts, and finally the pendulum that could keep the beat. Write each line down — one early slip plays every later bar out of time.

Everything from the chapter rides in it. 5★ under par here is the chapter crown. Count yourself in.