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a question about permutation in the digits in the decimal system

I dont get how it works but the method is:

think of a three didgit number with none of the same didgits ; 124 Reverse it ; 421 then take the smallest from the biggest ; 421-124=297 Then reverse your answer ; 792 Then add them together ; 792+297=1089

Now remember that number then do it again with a different number

867 768 867-768=099 990 990+99=1089

I dont get how it works

can you find a number that doesn't ???

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    See for example here: http://math.stackexchange.com/questions/183661/a-question-about-permutation-in-the-digits-in-the-decimal-system2012-09-05
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    it is so magical ;)2012-09-05
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    Now generalize it. What happens if you do the same problem with, say, appropriate four-digit numbers?2012-09-05

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