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The diagnostic bisects the knowledge graph. Each answer places whole regions of prerequisite topics at once rather than one topic at a time, so it usually finishes in about 15 questions. It continues up to 40 only when a strong run has not yet found your ceiling.
It is meant to feel slightly uncomfortable in both directions. Questions that are too easy tell it nothing, so it climbs until you miss, then settles.
You can retake it any time from Settings. A retake only adds to your profile; nothing already mastered is taken away.
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