Pick a three digit number. The three numbers used must be different (i.e. 123)
Reverse that number. (123 becomes 321)
Take the smallest three digit number from the largest.
(321 - 123 = 198)
Take the answer and reverse that number.
(198 becomes 891)
Add that number to the answer of the subtraction.
(891 + 198 = 1089)
The answer will ALWAYS be 1089!