\[\begin{array}{l}
b)\,\,\overline {a5} .3\overline {bc} = 7850\,\,\,\\
a \in \left\{ {1;\,\,2;\,\,3;\,\,.....;\,\,9} \right\}\\
b,\,\,c \in \left\{ {0;\,\,1;\,\,2;.....;\,\,9} \right\}\\
Ta\,\,\,co:\,\,\,15 \le \overline {a5} \le 95;\,\,\,300 \le \overline {3bc} \le 399.\\
+ )\,\,\,\overline {a5} = 15\\
\Rightarrow \overline {3bc} = 7850:15 \approx 523,33\,\,\,\\
\Rightarrow \overline {a5} = 15\,\,khong\,\,thoa\,\,man.\\
+ )\,\,\overline {a5} = 25 \Rightarrow a = 2\\
\Rightarrow \overline {3bc} = 7850:25 = 314\\
\Rightarrow b = 1;\,\,c = 4.\\
+ )\,\,\overline {a5} = 35\\
\Rightarrow \overline {3bc} = 7850:35 \approx 224,285\\
\Rightarrow \overline {a5} = 35\,\,khong\,\,thoa\,\,man.\\
Vay\,\,\,a = 2,\,\,b = 1,\,\,c = 4.
\end{array}\]
