1. Think of any 3 digit number as long as the first digit is 2 or more greater than the last one.
2. Reverse it.
3. Substract the smaller number from the bigger.
4. Reverse it.
5. Add numbers of the step 3 and 4.

You will alwasy get 1089.

Let's play it. I think of 643.

1. 643
2. 346
3. 643 - 346 = 297
4. 792
5. 297 + 792 = 1089