a) \(\overline{bccd}-\overline{abc}=\overline{ab}\)
\(\overline{abc}+\overline{ab}=\overline{bccd}\)
\(\overline{a}\times100+\overline{b}\times10+\overline{c}+\overline{a}\times10+\overline{b}\times1000+\overline{c}\times100+\overline{c}\times10+\overline{b}\)
\(\overline{a}\times110-\overline{b}\times990=\overline{c}\times109\)
\(110\times\left(\overline{a}-\overline{b}\times9\right)=\overline{c}\times109\)
\(\Leftrightarrow\hept{\begin{cases}\overline{a}=9\\\overline{b}=1\\\overline{c}=0\end{cases}\Leftrightarrow\overline{abc}}=910\)
