a: \(\overline{abc}\) ⋮7
=>100a+10b+c⋮7
=>2a+3b+c+98a+7b⋮7
=>2a+3b+c⋮7
b: \(\overline{abc}-\overline{deg}\) ⋮13
=>\(-\overline{abc}+\overline{deg}\) ⋮13
=>\(-\overline{abc}+\overline{deg}+1001\cdot\overline{abc}\) ⋮13
=>\(1000\cdot\overline{abc}+\overline{deg}\) ⋮13
=>\(\overline{abcdeg}\) ⋮13




