[tex]\displaystyle \\
\frac{\overline{ab0} - \overline{ba}}{99}= \frac{100a + 10b +0 - (10b+a ) }{99}= \\ \\
= \frac{100a + 10b +0 - (10b+a ) }{99}= \\ \\
= \frac{100a + 10b +0 - 10b-a}{99}= \frac{100a -a}{99}= \frac{99a}{99}= \boxed{a} \in N ~~\text{(fiind cifra)}[/tex]