Subtraction of a number I have a number $x$. If I remove the last digit, I get $y$. Given $x-y$, how can I find $x$?
For example x=34 then y=3 given 34-3=31, I have to find 34.
if x=4298 then y=429 , given 4298-429 = 3869 . how can I find 4298 from given 3869?

Clément Guérin's answer is correct as far as it goes. But the fact is, you can't always recover $x$ unambiguously from $x-y$. This is obvious if you think about it: $x-y$ is always less than $\frac{9}{10}x + 1$; so $x \mapsto x-y$ can't be injective.
For instance, $10$ and $9$ both map to $9$; and $54970$ and $54969$ both map to $49473$.
In general, if $x-y$ is divisible by $9$, then there are two solutions for $x$; otherwise Clément Guérin's answer gives a unique solution.
