Why does this work?
Use some algebra to prove it.
Let n = the number in the upper left corner of the 3 x 3 square.
Then the other eight numbers in the square are represented as follows:
n
|
n + 1
|
n + 2
|
n + 7
|
n + 8
|
n + 9
|
n + 14
|
n + 15
|
n + 16
|
The sum of the nine numbers is 9n + 72 = 9(n + 8).
So, when you divide by 9, you get the middle number (n + 8),
and when you subtract 8, you get n (the upper left number).