This is a classic nine-square problem. There is a jingle in Jiugongge, which also appeared in The Legend of the Condor Heroes. Wear nine shoes, one, three left and seven right, two or four shoulders, and six or eight is enough.

Written in the form of Jiugong is

|2|9|4|

|7|5|3|

|6| 1|8|

The simple way to understand it is to add the numbers of 1~9, and 1+2+3+4+5+6+7+8+9=45. According to the meaning of the question, the three rows of numbers should be the same, so the sum of each row and column should be 45/3= 15.