A library crew can shelve books in six hours. Another crew can do the job in four hours. If the first crew begins and is joined by the second crew one hour later, how many hours will it take to finish the job?

Answer 1

1st crew did (1/6)job
that leaves (5/6)job to be done with combined crews.

combined rate = job/6 + job/4 = 5job/12

time to do rest of job = (5/6)job ÷ (5job/12)
= 2 hours

It will take another 2 hours to finish