Question 107977
The trick is to invert things at first and turn them right side up at the end.
John can do 1/30 of the job in 1 day.
1/30+h=1/10
h=3/30-1/30
h=2/30=1/15 Harry can do 1/15 of the job in one day or the entire job in 15 days.
Ed