1089

Via Cut-The-Knot, a magic trick from David Acheson:

http://www.cut-the-knot.org/Curriculum/Arithmetic/S1089.shtml

• Take a three digit number (e.g. 602).
• Reverse it to get another three digit number: 206.
• Subtract the smaller from the larger: 602 - 206 = 396.
• Reverse this number, and add it to the answer obtained: 693 + 396 = 1089.
• Now try with another three digit number. What do you notice?

and a couple of follow-up questions:

• Can you explain it?
• Are there any numbers for which it doesn't work? Why not?