It takes 8 hours for one crew to clean an office building. It takes a second crew 10 hours to clean the same building. If the first crew works 2 hours and leaves and the second crew takes over to complete the job, how long will it take the second crew to finish cleaning the building?

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  1. 1/8-1 hour
    2/8- 2 hours

    x=7.5 hrs

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  2. let the work to be done be 1 unit

    rate of first crew = 1/8
    job done by first crew in 2 hours = 2(1/8) = 1/4
    job left to be done = 1 - 1/4 = 3/4

    rate of second crew = 1/10
    time needed for 2nd crew to finish
    = (3/4) / (1/10)
    = 30/4 hrs or 7.5 hrs

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