Question 830765
1-4/15-4/20=8/15 left to do
or 
7/15x=4
x=60/7 
x = 8.5714 days to to the whole job
4.5714 days more work
or
1/15+1/20=1/x
x=60/7
x = 8.5714 days to to the whole job
4.5714 days more work