Question 231488
A new printer can run off 1,500 workbooks in 8 hours, while an older printer takes 12 hours to run the same load. If both run at the same time to finish 1,500 workbooks, how many hours would it take.
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Call 1500 the job.
The new printer does 1/8 of the job per hour.
The old one does 1/12 of the job per hour.
Together they do 1/8 + 1/12 per hour = 3/24 + 2/24
= 5/24 job per hour.
--> 24/5 hours to to the job
= 4.8 hours
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There's a shortcut:  product over sum
= 12*8/(12+8) = 96/20
= 4.8 hours