\[V_{1} - скорость\ первой\ трубы;\]
\[V_{2} - скорость\ второй\ трубы;\]
\[1 - вся\ работа.\]
\[(1) - (2):\ \ \ \frac{8}{4V_{2}} = 8\]
\[\text{\ \ \ \ \ \ \ \ \ \ \ \ \ \ \ \ \ \ \ \ \ \ }\frac{2}{V_{2}} = 8\]
\[\ \ \ \ \ \ \ \ \ \ \ \ \ \ \ \ \ \ \ \ \ 8V_{2} = 2\]
\[\text{\ \ \ \ \ \ \ \ \ \ \ \ \ \ \ \ \ \ \ \ }V_{2} = \frac{2}{8} = \frac{1}{4}\]
\[t = V_{2}\ ;\ \ \ \ \ \ \ \ t = 1:\]
\[\frac{1}{4} = 4\ ч.\]
\[Ответ:4\ ч.\]