If it's worth saying, but not worth its own post, then it goes here.
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This is not the only case. For instance, the tops of the towers at heights 11, 17, 23 are collinear (both height differences are 6, both pairs are 2 primes apart).
Even if it turns out not to be relevant to the solution, the question should specify what happens in such cases.
Yes. You are right, I thought I might be wrong about this.
Okay. If they are collinear, then they are visible.