\[x - рабочих;\]
\[y - дней.\]
\[\left\{ \begin{matrix} - 6x + 5y = 30\ \ \ \ \ (1) \\ 10x - 5y = 50\ \ \ \ \ \ (2) \\ \end{matrix} \right.\ \]
\[(1) + (2):\ \ 4x = 80\ \ \ \ \ \ \ \ \]
\[\text{\ \ \ \ \ \ \ \ \ \ \ \ \ \ \ \ \ \ \ \ }x = 20.\]
\[Ответ:20\ рабочих.\]