Infinity...the answer would keep adding up, and it would be infinity. There would be no logicle way to add this up, no calculator or nothing. This would take years to solve without a calculator or even with it.
Arax, that last equasion equals 45 XP Now that you put it like that, the wording of the riddle does seem like all one digit numbers. Then the answer would be 45, but I think "allnumbers" is right! :O
I mean if you think about this being a riddle and not a math question, I would think that something silly like adding 1-9 together would be the answer. As Arax points out.
It is all the numbers, he didn't ask for negative numbers or anything.