Choose a three-digit number so that the first and third digits differ two or more units:

1. Write the number from right to left
2. Do the subtraction of the larger number by the smaller
3. Write the number obtained in point 2 from right to left and add it to the number also obtained in point 2.

And the result is 1089.

Example: Consider the number 503.

1. 305
2. 503 – 305 = 198
3. 198 + 981 = 1089