I reopened the question because I didn't see a compelling reason to close it.
Our applicable policies are:
When a problem dump should be closed isn't something we have a policy for; we've discussed it a few times, but with the usual level of participation on this meta site, no position has raised to the level of representing anything like community consensus. We certainly don't have a policy that anything that looks like a problem dump should be closed — that would be problematic since there's no consensual definition of “problem dump”.
When is it useful to close a problem dump?
- When answering would require too much effort, because the exercise requires a long solution.
- When the problem is made up in such a way that showing a solution won't help anyone other than if they have to hand in this exact homework assignment.
- When the problem is not particularly interesting and answers can be found readily so it isn't worth making the effort of writing one here.
For this question, too broad doesn't apply, the solution is fairly concise. The second reason doesn't apply either, it combines basic notions in a non-trivial but non-contorted way. (In fact I wouldn't be surprised if the question was a duplicate, but if it is I didn't find it. I wish someone would invent a semantic search engine!) Finally, when a question has already been answered satisfactorily, the third reason becomes moot.
Is there a compelling reason to close this question? Not that I can see. Is the web better off with or without complete_idiot's answer? I thought with, hence the question should remain open.
If we're to define a new policy regarding homework-type questions (do you want to ban undergraduate-level questions like Physics?), we need to get a discussion on meta with more than a handful of votes.
I'll re-close the question if you give me a good reason.
Apologies for not editing earlier, I didn't have time.
On an unrelated note, the reason you can't vote to close is because you've already voted to close once, not because the question was reopened by a moderator. You're now the only user on this site who can't vote to close this question. Since the question has received four votes from other users, I cast the fifth on your behalf.