\[\begin{array}{l}
\overline {a765b} \,\,\left( {a \ne 0} \right)\,\,\,chia\,\,het\,\,cho\,\,5 \Rightarrow b \in \left\{ {0;\,\,5} \right\}\\
+ )\,\,\,\,Voi\,\,\,b = 5 \Rightarrow so\,\,\,can\,\,tim:\,\,\,\overline {a7655} \\
Ta\,\,\,\,co\,:\,\,\,a + 7 + 6 + 5 + 5 = 23 + a\\
\overline {a7655} \,\,\, \vdots \,\,\,3 \Rightarrow \left[ \begin{array}{l}
23 + a = 24\\
23 + a = 27\\
23 + a = 30
\end{array} \right. \Leftrightarrow \left[ \begin{array}{l}
a = 1\\
a = 4\\
a = 7
\end{array} \right.\\
\Rightarrow So\,\,\,can\,\,tim:\,\,\,17655;\,\,\,47655;\,\,\,77655.\\
+ )\,\,Voi\,\,\,b = 0 \Rightarrow so\,\,\,can\,\,tim:\,\,\,\overline {a7650} \\
Ta\,\,\,\,co\,:\,\,\,a + 7 + 6 + 5 + 0 = 18 + a\\
\overline {a7655} \,\,\, \vdots \,\,\,3 \Rightarrow \left[ \begin{array}{l}
18 + a = 18\\
18 + a = 21\\
18 + a = 24\\
18 + a = 27
\end{array} \right. \Leftrightarrow \left[ \begin{array}{l}
a = 0\,\,\,\,\left( {ktm} \right)\\
a = 3\\
a = 6\\
a = 9
\end{array} \right.\\
\Rightarrow So\,\,\,can\,\,tim:\,\,\,37650;\,\,\,67650;\,\,\,97650.
\end{array}\]
