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					Originally Posted by Ghoulish Delight
					
				 
				Well, if X=the first 3 digits of your phone number and Y=the last 4, then your phone number can be represented as 10000X+Y 
 
So, the operations you perform look like this:
 
 (80X+1)*250+2Y-250 
___________________ 
2 
   Simplifying the numerator, you get 20000X+250+2Y-250. 
  
The +250 and -250 cancel out, leaving you with 20000X+2Y. Divide that by 2 and you get 10000X+Y, which of course is your phone number.  
			
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 My brain hurts just looking at that.