Any four-digit number is made up of four digits. After the four digits are reversed, a maximum number and a minimum number are formed, and the two numbers are subtracted to get a new number. The above process is repeated at most 7 times, and 6 147 is obtained, unchanged.

There is a similar result, but it is not 6 147, but 6 174. This is correct. In the preface of Primary School Teaching Design (Mathematical Science Edition)No. 12 in 2007, there are three digits of 495 and four digits of 6 174.

For example: take 7642 as an example: 7642-2467 = 5175; 755 1- 1557=5994; 9954-4599=5355; 5553-3555= 1998; 998 1- 1899=8082; 8820-0288=8532; 8532-2358=6 174; 764 1- 1467=6 174.-