This arithmetical stunt is not difficult when you know how, says Jason England.

Magicians often include acts that are not strictly magic, but leave their audiences feeling that they have witnessed something impossible. Memory stunts, unusual scientific demonstrations, playing chess blindfolded and rapid mental mathematics are some examples.

Successful stunts of this kind suggest the performer has an enhanced talent. In some cases, this is true – they may have an extraordinary memory or be a chess grandmaster. But in most cases the performer is using a system. These systems can be simple, or can require tremendous skill – but they are still easier than doing the stunt without any system at all. Read more..

https://cosmosmagazine.com/mathematics/how-solve-magic-square