I wrote this in a forum in different words, but I forgot what the forum is. Now I write here for sharing. Let’s see.

1. Write a three number with each figure different. Example: 345

2. Write the same number backwards 543

3. Now subtract the smaller number from the larger one 543-345=198

4. Turn the answer backwards and add this number to the answer 198+891=1089

You can try with different number and we know that the answer is always 1089.

If your answer should have only two figures, add a zero to make three figures like 99=099

(312-213) =099