What's the appropriate education level for the following concise but non-trivial geometry problem?
Points $A$, $B$, $C$ are collinear; $\|AB\|=\|BD\|=\|CD\|=1$; $\|AC\|=\|AD\|$.
What is the set of possible $\|AC\|$ ?
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The mean of the elements of the set of solutions is $\approx 1.08$.
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