<p>My opinion on this might be in the minority, but I am going to post this anyway, as a suggestion.  If this is inconsistent with policy, consider it a suggestion for a change of policy.  As a premise to my opinion, I note that there is a general consensus that "textbook problems" are valuable problems for statistical learning, and so we do not close or ignore these problems.  Our present practice for self-study questions is to require the user to show what they have done, and where they are stuck, and then provide a "hint" for further progress.</p>

<p>I agree that it is sensible not to provide contemporaneous solutions to self-study problems, since this gives the user a facility to have others do their homework for them, and it encourages students to post homework or assessment problems.  However, setting aside the issue of contemporaneous answers, I see no particular problem with this site gradually accruing a large repository of <em>full worked solutions to textbook problems</em>.  That actually seems to me like a valuable resource, for exactly the same reason that worked solutions in textbooks and courses are valuable resources for learning.  As I see it, the issue is to avoid giving <em>contemporaneous</em> worked solutions to the student for their homework/assessment.  Thus, once the question is "old" (e.g., at least six-months old, which is past the assessment point) I think it should be okay to post a full worked solution to the problem.  Indeed, at that point, I think it is <em>desirable</em> to give a full worked solution, to build up a repository of worked solutions to simple problems.</p>

<p>This proposal retains the property that it does not incentivise students to post homework or assessment problems, to bypass solving these themselves.  By imposing a delay of six months on posting worked solutions, we ensure that the full solution comes too late to be used as submitted homework or assessment in a course.  (I actually think this issue is a bit overblown anyway, since students almost always need to replicate performance on an in-person exam, and SE solutions cannot be used as a substitute in that environment.)  At the same time, it allows us to build up a good repository of worked solutions to simple "textbook problems" which is a useful pedagogical resource.</p>
