SOLUTION: A roofer requires 12 h to shingle a roof. After the roofer and an apprentice work on a roof for 9 h, the roofer moves on to another job. The apprentice requires 12 more hours to fi

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Question 1176327: A roofer requires 12 h to shingle a roof. After the roofer and an apprentice work on a roof for 9 h, the roofer moves on to another job. The apprentice requires 12 more hours to finish the job. How long would it take the apprentice. Working alone, to do the job?
Answer by ikleyn(52798) About Me  (Show Source):
You can put this solution on YOUR website!
.

Let x be the time for the apprentice to do the entire job working alone.


Then, based on the given info, we can write this equation


    9%2F12 + 9%2Fx + 12%2Fx = 1.

or

    3%2F4 + 21%2Fx = 1,

    21%2Fx = 1 - 3%2F4

    21%2Fx = 1%2F4

    x = 21*4 = 84 hours.


ANSWER.  The apprentice needs 84 hours to complete the job working alone.

Solved.