math
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.
2/(2x)+1/(2x)=1/6
3/(2x)=1/6
Cross multiply
2x=18, x=9
So father does the wok in 9 hours. 
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