Answer:
[tex]8 \frac{4}{7}[/tex]
Step-by-step explanation:
The computation of the number of weeks taken for empty the lake is shown below:
Given that
Lake empty in 6 weeks
The River could fill the lake in 20 weeks
Let us assume the combined lake be x
So, here the equation is
[tex]\frac{1}{6} - \frac{1}{20} = \frac{1}{x}[/tex]
[tex]\frac{20 - 6}{6\times 20} = \frac{1}{x}[/tex]
[tex]\frac{14}{6\times 20} = \frac{1}{x}[/tex]
After solving this
x = [tex]\frac{60}{7} \\\\= 8 \frac{4}{7}[/tex]
hence the number of weeks that take for empty the lake is [tex]8 \frac{4}{7}[/tex]