Here we have a problem about NP-hardness for magic squares, posted by @levanovd. At first I do consider it as a home-work level question, and suggest the OP to wait for answers on Math SE. But after trying to solve the problem for a while, although there is a solution to the problem, I found it non-trivial. So I flipped my opinion and suggested to keep to question open.
It is my fault to give suggestions before having a clear thoughts on the level of the question, and I feel sorry to @levanovd.
In order to prevent the same thing happened again, here I want to ask:
What kind of NP-hardness problems are considered on/off-topic?
The judge may be depended on each user, and by the number of closing votes we should solve this problem with no difficulties; but currently there aren't too many users have the power to give the vote, so maybe some rules will help us to decide what kind of suggestions should be given to the OP.