[tex]\it \dfrac{a}{b}=\dfrac{a}{10}+b|_{\cdot10b}\ \Rightarrow 10a=ab+10b^2 \Rightarrow 10a-10b^2=ab \Rightarrow ab\in M_{10} \Rightarrow \\ \\ \Rightarrow ab\in\{10,\ 20,\ 30,\ 40\} \\ \\ (a,\ b) \in\{(2,\ 5);\ (5,\ 2);\ (4,\ 5);\ (5,\ 4);\ (6,\ 5);\ (5,\ 6);\ (8,\ 5);\ (5,\ 8)\}[/tex]
