Choose a 4-digit number (except 1111, 2222, etc.) and do the following:

1) Put your digits in ascending order

2) Put your digits in descending order

3) Subtract the smaller number from the larger number

Repeat steps 1, 2 and 3 for the result, and so on. What happens?

Let's try number 8415.

Digits in ascending order: 1458

Digits in descending order: 8541

Subtracting the smallest from the largest: 8541 - 1458 = 7083

Repeating the process with the number 7083:

8730 - 0378 = 8352

8532 - 2358 = **6174**

7641 - 1467 = **6174**

7641 - 1467 = **6174**

7641 - 1467 = **6174** (the repetition continuesâ€¦)

Incredibly, all 4-digit numbers that are not multiples of 1111 end up falling into this repetition.