[tex]\it 6)\\ \\ \overline{25a}\ \vdots\ 2 \Rightarrow a\in\{0,\ \ 2,\ \ 4,\ \ 6,\ \ 8\}\ \ \ \ (1)\\ \\ \overline{25a}\ \not\vdots\ 3 \Rightarrow a\not\in\{2,\ \ 8\}\ \ \ \ (2)\\ \\ (1),\ (2) \Rightarrow \overline{25a}\in\{250,\ \ 254,\ \ 256\}[/tex]
