Let me expand upon the argument that I first put forth in the comments on that question.
Why the homework policy applies
The first sentence of the homework policy specifies what sorts of questions it applies to:
A "homework question" is any question whose value lies in helping you understand the method by which the question can be solved, rather than getting the answer itself.
This is intentionally somewhat vague; it speaks to the intent behind the policy (see below for more on this intent) rather than giving an objective procedure for determining whether a question is homework-like, because frankly, we haven't been able to come up with such a procedure. In practice we rely on our high-reputation members' intuition for how homework questions are written to determine which questions it should apply to.
However, the relevant part of the question we're discussing reads
An electric kettle rated 220V, 2000W needed 10 minutes to boil water when it is half filled with water in Singapore where the output voltage is 220V. Estimate the amount of time needed to do the same task if the kettle was brought to the USA where the output voltage is 110V.
...
Can anyone explain how 40 minutes was derived in the question above?
I hope we all agree that this question does satisfy the criterion presented in the first sentence of the policy, and thus that the homework policy is applicable.
Why the question was closed
The relevant part of the homework policy for this point is this (emphasis added):
We expect you to narrow down the problem to the particular concept that's giving you trouble and ask about that specifically. That produces a question that is more relevant to others who might be having the same problem,
And let me quote again the core question from the post being discussed:
Can anyone explain how 40 minutes was derived in the question above?
I think it's quite clear that this core question, considered in isolation, is not about a conceptual issue that was encountered while trying to solve the problem, it's asking for a complete explanation of the solution. But of course there was more in the post than just that core question and the quotation of the homework problem. Between them is the following:
My answer to this question is Option 3 (20 minutes) because I thought that since output voltage is the USA is 2 times less than that of Singapore, it would take twice as long to do the same task of boiling water half filled. However, the answer key was Option 4 (40 minutes).
However this part does not actually ask anything, nor does it modify what the core question is about. Accordingly it doesn't change the fact that the question is asking for a complete solution.
There is also this piece:
P.S. I don't quite understand what it is meant by " An electric kettle rated 220V, 2000W" ??? I get that 2000W means the kettle uses 2000J/s and V means work done by kettle/charge. But what is the difference between the 2 (power and charge of the kettle)? And how does the V of kettle differ from output voltage?
which I don't believe is relevant for two reasons: first of all, it shouldn't be possible to sneak a request for a complete solution through by also asking something else in the same question. To implement that, posts which primarily ask for a complete solution but include some other supplementary questions should still be closed. Besides, "P.S." explicitly indicates that the following material is not the main focus of the question.
Regarding these two points,
(1) the OP attempted the question and got it wrong (2) the OP has the answer key and posted the correct answer.
I would say again that the homework policy requires an asker to narrow down their question to the specific conceptual issue that is giving them trouble. Both points (1) and (2) are true but they do not change the fact that the OP in this case is not asking about a specific conceptual issue.
Why the answer was deleted
Simply, this excerpt from the homework policy covers it:
If someone posts an answer to a homework-type question that gives away a complete or near-complete solution, in most cases it will be temporarily deleted.
I maintain that the answer which I deleted gives away a near-complete solution, in that it explains how one would derive the given answer of 40 seconds. Yes, it does identify that the question is a poor one, but that doesn't change the fact that the answer gives away a near-complete solution. (For now I'm refraining from posting that near-complete solution here, but if it becomes important I'll edit it in.)
What is the intent of the homework policy?
All the above discussion has addressed why (I think) the question should be closed and the answer deleted under the letter of the homework policy, but not under its spirit. In particular, let's consider the argument that, regardless of what the HW policy actually says, its intent is to prevent cheating and a complete answer isn't conducive to cheating in this case. I don't agree, for two reasons. First, because the value of the problem lies in the OP working through it step by step, doing as much of it as possible, so as to learn the method. By giving away a complete explanation of how the answer is achieved, we rob the OP of that value. And second, because any time we provide a complete or near-complete answer to a homework-like question, it pushes our site's reputation a tiny bit toward that of a place where people can come and get their homework problems explained to them. We don't want that. This is supposed to be a site primarily geared toward physics research and applications, not a homework help site. And we already get too many homework-like questions despite not having the reputation of a site that welcomes them.
This is expressed in other words in the following quote from a Math Meta post, which is also quoted in our homework policy:
Providing an answer that doesn't help a student learn is not in the student's own best interest, and if a solution complete enough to be copied verbatim and handed in is given immediately, it will encourage more people to use the site as a free homework service. In the spirit of creating a lasting resource of mathematical knowledge, you may come back after a suitable amount of time and edit your response to include a more complete answer. Or even better, the student can post his own correct answer!
