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28 February, 12:30

Juan takes 15 hours to do a job that his father can do in 6 hours. If juan works for 4.5 hours before his father joins him, how long will it take the two of them working together to finish the job?

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  1. 28 February, 14:34
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    Work it exactly like the other one with Wilda and Karla.

    Juan takes 15 hours to do a job ... he does 1/15 of it in an hour.

    His father can do it in 6 hours ... he does 1/6 of it in an hour.

    Juan works alone for 4.5 hours. In that time, he does 4.5/15 = 0.3

    of the job. When his father joins him, there's only 0.7 of it to finish.

    Working together, they do (1/15 + 1/6) = (2/30 + 5/30) = 7/30 of the job

    each hour.

    How many times do they have to do 7/30 of the job in order to finish

    the 7/10 of it that remains? That will be the number of hours they need.

    (7/10) / (7/30) = 7/10 x 30/7 = 3 hours.

    Then they can knock off while it's still daylight, and go for a swim.
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