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 Post subject: GMAT Rate
PostPosted: Thu Nov 10, 2011 5:51 pm 
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Joined: Sun May 30, 2010 3:15 am
Posts: 424
It takes 50 identical printing presses 8 hours to print 500,000 papers. How long would it take 40 of these presses to print the same total?
A. 9 hr, 36 min
B. 10 hr
C. 12 hr 24 min
D. 12 hr, 40 min
E. 13 hr, 20 min

(B) If it takes 50 presses 8 hours to print 500,000 papers, then it takes 50 × 8 = 400 press-hours to make the total. If there were only 40 presses, and 400 press-hours were needed to print the papers, then each press would need to work for 400/40 = 10 hours to finish the total.
The correct answer is choice (B).
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Your answer to this question is wrong.

How can it take 400 hours to make 500k when it is stated in the problem that it takes 8 hours?

The math... step by step is 500,000 divided by 8 = 50 machines make 65,000 per hour.

65,000 / 50 = 1300 per hour per machine

1300 × 40 = 52,000 per hour for 40 machines

500,000 / 52,000 = 9.6 or 9 hours 36 minutes


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 Post subject: Re: GMAT Rate
PostPosted: Thu Nov 10, 2011 6:00 pm 
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Joined: Sun May 30, 2010 2:23 am
Posts: 498
Quote:
The math... step by step is 500,000 divided by 8 = 50 machines make 65,000 per hour.
500,000 / 8 = 62,500. If you do the rest of the calculations, you'll see that it'll lead you to the answer choice B, 10 hr.

Quote:
How can it take 400 hours to make 500k when it is stated in the problem that it takes 8 hours?
Here we deal NOT with 400 hours, but with 400 press-hours. It's often useful to deal with man-hours, car-hours, etc.
The value, 400 press-hours, tells us that it would take 1 press 400 hours to get the job done. If 400 presses do it, it will take them 1 hour. If 4 presses do it, it will take them 400/4 = 100 hours. And so on.


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