This is the one that decides Question 14 to 16. The tool is one you know — a geometric series, a recurrence, the triangle inequality — but the question does not name it. The page starts down the wrong road, runs out of time, and the part is worth zero.
It is not fixed by more content. It is fixed by practising the first minute: what are they asking, what did I just get given, what shape is this, which of my moves fits — and which don't.