Question 1119209
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The correct answer is 9 hours for the plumber to make the job working alone.


<U>Solution</U>


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Let  r be the rate of of work of the apprentice.


Then the plumber's rate of work is 2r.


After working 3 hours alone, the plumber completed  3*(2r)  pars of the entire job.


When the apprentice joined him, they worked 4 more hours and made 4*(2r+r) = 12r  parts of the job.


For the entire job you have this equation


    3*(2r) + 12r = 1,   or

    18r  = 1.


It means  r = {{{1/18}}},  i.e.  apprentice can make the job in 18 hours working alone.


Since the plumber works two times faster, he needs only 9 hours.
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Solved.