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?

Let $d=x-y$ be the known difference.  Since $x=10y+b$ with $0\le b\le9$, then  $d=9y+b\equiv b$ mod $9$.  Thus $b$ can be retrieved uniquely from $d$ by casting out $9$s, unless the result of doing so is $0$, in which case $b$ is either $0$ or $9$.  When the result is unique, as it is for $d=3869\equiv8$ mod $9$, you can retrieve $y=(d-b)/9$, and then get $x=10y+b$.
The simple example $100-10=90=90-0$ illustrates what happens when $d\equiv0$ mod $9$.
