This is the boss. No calculator. Tonight the coupon's area, density and hours have already been collapsed into one number: mils per year is the milligrams lost divided by 120. The pipe is 280 mils of wall today against a 100 mil retirement thickness, so there are 180 mils to spend. Everything has been chosen to fit in your head.

One water, one coupon, one pipe, and every answer feeds the next: the two pH values give the Langelier index, the same two give the Ryznar index, the coupon gives a rate in mils per year, the 180 mils divided by that rate gives the years left, and those years are held up against the turnaround date. Write each line down as you go — one slip at the top rides all the way to the verdict.

Note what the last line asks for. It does not want another number; it wants a decision: does this wall reach the turnaround, yes or no. And note which line earns you that decision. The indices tell you what kind of water you are standing in and whether it will build a protective film. They do not give a rate and they never have. The coupon does, and lines four and five are built on the coupon alone. Reading an index as a rate is the one mistake this whole chapter has been aimed at.

Everything in the chapter rides in this one survey. 5★ under par here is the chapter crown. Clock in.