This is the boss. No calculator. Every draw tonight is chosen to fall out in your head: the water contents differ by a round tenth, the depletions are halves and fifths, the crop coefficients land the crop use on a whole millimetre a day, and the mainline flow is given in cubic metres per hour so the depth identity is the only bridge you need.

That identity, once more, because everything tonight rides on it: 1 mm over 1 ha is 10 m³, exactly.

Two questions, and inside each one every answer feeds the next. The first works down from the soil: the two water contents give the reserve, the reserve and the allowable depletion give the allowance, the crop coefficient and the reference ET give the daily spend, and the allowance divided by the spend gives the interval. The second works down from the depth: efficiency turns the net depth into a gross one, the gross depth over the set area turns into a volume, and the volume over the mainline flow turns into hours. Write each line down — one slip at the top rolls all the way to the pump.

And watch what the last line of each question actually wants. Not another number: a decision. Does the set clear its window, yes or no. The arithmetic exists only to earn you the right to make that call — and the trap the whole chapter has been building is right here, in the efficiency step, where the depth that looked comfortable grows by a quarter and quietly overruns the night.

5★ under par here is the chapter crown. Clock in.