[Curtis1993]

I need help setting up this problem. A library crew can shelve books in 6 hours. Another crew can shelve books in 4 hours. If the first crew begins and is joined by the second crew on hour later, how many hours will it take.

[ArcSine]

Maybe this'll help you get the ball teed up: The first crew can do 1/6 of one complete job per hour (and they worked alone for the first hour--so how much of the job was completed that first hour?). The second crew can do 1/4 of one complete job each hour, so working together they're knocking out 1/6 + 1/4 = 5/12 of one complete job per hour.

Now, if "1" is the number of hours the first crew worked alone, and "x" is the number of hours they worked together, you'll need

(1 hour times completion rate of first crew) + (x hours times completion rate of both crews) to be equal to 1 complete job.

That's sort of a verbal rendering of your needed equation. You can take over. (And remember that the total time to complete the job is the x hours they worked together, plus the one hour the first crew worked alone.)

[Curtis1993]

Thank you so much for your help. I got it!!