\[\boxed{\text{743\ (743).}\text{\ }\text{Еуроки\ -\ ДЗ\ без\ мороки}}\]
\[\textbf{а)}\ \overline{\text{abc}} + \overline{\text{cba}} =\]
\[= 100a + 10b + c + 100c + 10b + a =\]
\[= 101a + 20b + 101c\]
\[\textbf{б)}\overline{\text{\ abc}} + \overline{\text{bc}} =\]
\[= 100a + 10b + c + 10b + c =\]
\[= 100a + 20b + 2c\]
\[\textbf{в)}\overline{\text{\ abc}} - \overline{\text{ba}} =\]
\[= 100a + 10b + c - 10b - a =\]
\[= 99a + c\]
\[\textbf{г)}\ \overline{\text{abc}} - \overline{\text{ac}} =\]
\[= 100a + 10b + c - 10a - c =\]
\[= 90a + 10b\ \]