Comparing apples with apples, page-17

(1) Weight the trench lengths (each trench length / total of 867.5)

(2) Multiply the weight by the flow rate in m3/hour

(3) Add up the numbers in (2). This is the weighted average flow into the 867m of trench = 28.28m3/hour

(4) Solve it for 50m trench length: divide 28.28 / 17.35 = 1.63m3/50m/hour.

I can't see how this can be wrong? Forgetting that some are posing the argument that a weighted average is the incorrect approach. I think you'll find that your looking at the already extrapolated number; you've already built in the same error.

It might be an error at all, happy to be proved wrong!