\[\boxed{\text{680\ (680).}\text{\ }\text{Еуроки\ -\ ДЗ\ без\ мороки}}\]
\[\left\{ \begin{matrix} x_{1} = 1,2\ \ \ \ \ \ \ \ \ \ \ \ \ \ \ \ \ \ \ \ \ \ \\ x_{1}\left( x_{1} + 0,1 \right) = 0,5c \\ \end{matrix} \right.\ \text{\ \ \ \ }\]
\[1,2 \cdot (1,2 + 0,1) = 0,5c\ \ \ \ \ \ \ |\ :0,5\]
\[c = 1,2 \cdot 1,3\ :0,5 = 3,12\]
\[Ответ:c = 3,12.\ \]