You can get the same sum of 1,089 every time you add certain numbers together:

1. Take any three-digit number in which the first digit is larger then the last digit (654).
2. Reverse the number and subtract the smaller number from the larger one (456; 654 - 456 = 198).
3. Reverse the result and add this number to the result (198 reversed = 891 + 198 = 1,089)
4. As shown above, the answer is 1,089 every time you use the procedures as indicated.