Question 1185686
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 4 skilled workers can complete a job in 5 days. 5 semi-skilled workers can complete the same job in 6 days. 
How long does it take 1 skilled work and 1 semi-skilled worker to complete the same job if they work together?
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Rate of work of one skilled worker is  {{{1/(4*5)}}} = {{{1/20}}}  of the job per day.


Rate of work of one semi-skilled worker is  {{{1/(5*6)}}} = {{{1/30}}}  of the job per day.


When one skilled worker and one semi-skilled worker work together, their combined rate of work is


    {{{1/20}}} + {{{1/30}}} = {{{3/60}}} + {{{2/60}}} = {{{5/60}}}  = {{{1/12}}}  of the job per day.


It means that one skilled worker and one semi-skilled worker will complete the job in 12 days working together.    <U>ANSWER</U>
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Solved.