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This formula is much easier than the standard 10x-x to do in your head. I have been able to do 3 digit numbers with this trick easily, and I'm generally not that great at doing math in my head. Anyway, he it is:
http://dratsab.deviantart.com/art/Multiplication-of-9-s-Trick-85433340
http://dratsab.deviantart.com/art/Multiplication-of-9-s-Trick-85433340