\[\boxed{\text{1220.\ }\text{еуроки}\text{-}\text{ответы}\text{\ }\text{на}\text{\ }\text{пятёрку}}\]
\[b = a + c\]
\[\frac{\overline{\text{ac}}}{\overline{\text{abc}}} = \frac{10a + c}{100a + 10b + c} =\]
\[= \frac{10a + c}{100a + 10 \cdot (a + c) + c} =\]
\[= \frac{10a + c}{100a + 10a + 10c + c} =\]
\[= \frac{10a + c}{110a + 11c} =\]
\[= \frac{10a + c}{11 \cdot (10a + c)} = \frac{1}{11}\]