Question 1140465
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The slower computer takes 30 minutes to do the job alone.  So when the two computers together do the job in 10 minutes, the fraction of the job that the slower computer does is 10/30 = 1/3.<br>
That means in those 10 minutes the faster computer does 2/3 of the job.<br>
And that in turn means the amount of time it takes the faster computer to do the job alone is 3/2 times 10 minutes = 15 minutes.