posted by Irene Wu .
A father can do a job in a certain number of hours. His son takes twice as long to do the job. Working together, they can do the job in 6 hours. How many hours does it take the father to do the job alone?
Father does the work in x hours, or
father does 1/x of the work in one hour.
Son does 1/(2x) of the work in one hour.
Together, they do 1/x+1/(2x)=1/6 of the work in one hour.
So father does the wok in 9 hours.