This is the boss. No calculator — today the hardness factors round to $2.5$ and $4$, the k values arrive clean, and every number is chosen to fit in your head. One raw-water sample, start to finish, and every answer feeds the next: the meter's TDS, the titration's hardness, yesterday's jar test, and finally tomorrow's chlorine dose. Write each line down as you go — one early slip flows all the way to the pump.

Everything from the chapter rides in this one sample, and this bench is the site's home turf — treat it with the respect of a full water report. 5★ under par here is the chapter crown. Clock in.