Question 990815
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The other tutor's answer is incorrect.
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Two painters were working on a house. Painter A can complete the given job in 8
hours
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Painter A's rate is 1 job per 8 hours or 1/8 job per hour
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but painter B takes 10 hours.
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Painter B's rate is 1 job per 10 hours or 1/10 job per hour 
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They worked together for the first four hours, 
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Their combined rate is 1/8 + 1/10 = 5/40 + 4/40 = 9/40 job per hour.

In those 4 hours at that rate they did 4(9/40) or 9/10 of the job.

So 1/10 of the job remained to be done.
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but painter A left and told painter B to finish the job.
How long would it take for painter B to finish the job? 
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Let the answer be x hours.  Then rate times time = the remaining 1/10

B's rate is 1/10 jobs per hour, so the equation is rate times time = 1/10.

              (1/10)(x) = 1/10
                      x = 1 hour.

Edwin</pre>