Question 478529
A group of workers can do a job in 8 days.
 After this group had worked in 3 days, another group had joined.
Together they completed the job in three more days.
How long could the second group have done the job alone?
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Let x = time required by the 2nd group to do the job
Let the completed job = 1
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From the given information, we know that the 1st group worked 6 days & the 2nd group worked 3 days
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A typical shared work equation, each group does a fraction of the job, 
the two fractions add up to 1.
:
{{{6/8}}} + {{{3/x}}} = 1
multiply by 8x, results:
6x + 3(8) = 8x
24 = 8x - 6x
24 = 2x
x = {{{24/2}}}
x = 12 days for the 2nd group to do the job 
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See if this adds up:
6/8 + 3/12
3/4 + 1/4 = 1
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Did this method make sense to you?