<PRE><B>Question 172268</B>
A high-speed copier can copy a certain report in 10 hours. An older copier requires 15 hours to do the same task. How long would it take to copy the report with both copiers operating?
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You have to figure what part of the job each does per hour.
One does 1/10 per hour, the other 1/15 per hour.  Add those to get
(1/10) + (1/15) = 25/150 = 1/6
1/6 of the job per hour means it will take 6 hours.
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A shortcut for this type of problem (with 2 elements) is the product divided by the sum.
10*15/(10+15) = 150/25 = 6 hours.

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