The Number  26 Sep 2009
Find the number which when added to any other number does not change the sum of its digit to ultimately single digit.

Re: The Number  26 Sep 2009
It must be 9.

Re: The Number  26 Sep 2009
9 and 0 both
123 + 9 = 132 ...... 1+2+3 = 1+3+2 = 6 123 + 0 = 123 ...... 1+2+3 = 6 
Re: The Number  26 Sep 2009
9

Re: The Number  26 Sep 2009
Any number of 9s will do the trick, so 99 (+1001), 999 (+10001) etc will all work
198 will also work (2002), as will 297 (3003) and any "extension" (4004, 5005 etc, and 30003, 300003, etc). In short, the answer is any number of the form x*10^yx. nimesh is also correct because y=0. x doesn't have to be less than 10; 125000125 will also work for example (as will 1125: try it!). 
Re: The Number  26 Sep 2009
nice answer XPT :)

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