Question 949827
Uniform fill or work rates, using the lake as the unit of volume, for 1 LAKE.
Time is quantity of weeks.  Notice the filling rate uses less time than the emptying rate.  The rate for both together is the sum of the individual rates.  Understand that the two rates are of opposite signs.


{{{1/6-1/20}}}, LAKES per WEEK.


You are finally interested in the reciprocal of that.  How many weeks.