Question 949827
Lake empties at rate -1 lake/6wks=-1/6 lake/wk
Lake fills at rate lake/20wks=1/20 lake/wk
Combined rate: -1/6 lake/wk + 1/20 lake/wk=-10/60 lake/wk +3/60 lake/wk=-7/60 lake/wk
To empty the 1 lake at rate -7/60 lake/wk=1/(-7/60)=-(60/7)wks= 8 4/7 wks
ANSWER:  It would take 8 weeks and 4 days to empty the lake.