Question 980198
Oh darn it!  She's already done it for you before I could post this!
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To prevent students from merely turning in my answers to multiple choice
homework problems, instead of doing the exact same problems, I do a
problem EXACTLY similar to it, but changing the numbers.  So the problem
I will solve is this one. You can use it as an exact model for your problem.
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A can do a job in 10 hours. After A worked alone for 2 hours, B joined him.
Together they finished the job in 3 hours. How long would it take B to do
the job alone.
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Begin with this statement:
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A can do a job in 10 hours. 
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Therefore A's work rate is 1 job per 10 hours or 1/10 job per hour. 
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How long would it take B to do the job alone?
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Suppose it would take B x hours to do the job alone.

Then B's work rate is 1 job per x hours or 1/x job per hour
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After A worked alone for 2 hours, 
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The part of the job that A did in those 2 hours is found by RATExTIME

So in those 2 hours A did (1/10)(2) = 2/10 = 1/5 of the job.

That left 1-2/5 = 5/5-1/5 = 4/5ths of the job still to be done.
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B joined him. 
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So then their combined rate was the sum of their rates (1/10 + 1/x).
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Together they finished the job in 3 hours. 
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The part of the job that they did in those 3 hours is also found by
RATExTIME, and the part they did together must equal to the remaining 
4/5 of the job.  So the equation is 

             (1/10 + 1/x)(3) = 4/5

               3(1/10 + 1/x) = 4/5

                  3/10 + 3/x = 4/5

Multiply through by LCD of 10x

                     3x + 30 = 8x

                          30 = 5x

                           6 = x

So it would take B 6 hours to do the job alone

Now use this as an exact model to solve your problem.                                

Edwin</pre>