This is the boss. No calculator — the numbers are chosen to carry in your head, the way a kind examiner sets them: the tap runs at 20 °C so the climb is a clean 80 degrees, and 4.2 × 80 is just 4 × 80 plus a fifth of 80 — 336. One kettle, worked line by line, and every answer feeds the next, so write each one down as you go.

The whole chapter rides in it: the climb priced by $Q = mc\,\Delta T$, the plateau priced by $Q = mL$, and the legs added — plus one backwards bonus from the freezer. Say each formula aloud before you use it; the read-aloud IS the recall. 5★ under par here is the chapter crown. Put the kettle on.