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Take any three-digit number in which the first and last digits are different. Reverse the digits to get a new number, so now you have two numbers, and each one is the reverse of the other. Subtract the smaller from the larger to get a third three-digit number. (If subtracting gives you a two digit number, then please treat it as a three-digit number whose first digit is zero.) Now add this number to its reverse. The result will be 1089.
Why?
Example:
571-175 = 396,
396+693 = 1089
The webmaster and author of this Math Help site is Graeme McRae.