Question 603368
Joe can do a job twice as fast as Sam. 
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Let Sam's rate in jobs per hour be S.
Then Joe's rate in jobs per hour is 2S.
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Together they can do the job in 20 minutes. 
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Their combined rate is 1 job per 20 minutes = 1 job per 1/3 hour or 3 jobs per hour.

So the sum of their rates is 3 jobs per hour

S + 2S = 3

    3S = 3

     S = 1

So Sam's rate is S = 1 job per hour, so it'll Sam 1 hour to do 1 job 

and Joe's rate is 2S = 2·1 = 2 jobs per hour, so it'll take Joe half
an hour (or 30 minutes) to do 1 job.

Edwin</pre>