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og math # 130

This topic has 8 expert replies and 121 member replies
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Shilpa77 Newbie | Next Rank: 10 Posts Default Avatar
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Post Sat Aug 23, 2014 9:49 am
Answer D : 22/15

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suahib Newbie | Next Rank: 10 Posts Default Avatar
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Post Thu Aug 28, 2014 12:32 am
rate ratio
1/12*(15)(18)/33
=15/22
as
rate*time= work
to get the time to complete 1 job
t=22/15

jaspreetsra Master | Next Rank: 500 Posts
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Post Tue Sep 23, 2014 9:24 am
resilient wrote:
hmm still not seeing the picture. What I am trying to grasp is why the flip of the combined rates. It doesnt make sense to me and goes against what is taught with mahattan gmat. confused

thank you
Printer Rate Time Work Done
X 1/12 12 1
Y 1/15 15 1
Z 1/18 18 1
Y+Z 11/90 *90/11 1

*Y+Z = 1/15 +1/18 =11/90
Ratio:
12: 90/11
=132:90
=22:15

akash singhal Master | Next Rank: 500 Posts Default Avatar
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Post Tue Apr 28, 2015 6:08 am
Answer is 22/15
Simple calculation already explained...

chanstee Newbie | Next Rank: 10 Posts
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Post Sat Jul 11, 2015 5:12 am
Answer is e 22/15

lb2012 Newbie | Next Rank: 10 Posts Default Avatar
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Post Tue Sep 01, 2015 1:55 am
D is correct

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Max@Math Revolution Legendary Member
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Post Wed Sep 02, 2015 9:39 pm
Forget conventional ways of solving math questions. In PS, IVY approach is the easiest and quickest way to find the answer.


working alone, printers x,y, and z can do a certain printing job, consisitning of a large number of pages, 12, 15, and 18 hours, respectively. What is the ratio of the time it takes printer x to do the job, working at its rate, to time it takes printers y and z to do the job, working together at their individual rates?

a. 4/11
b.1/2
c. 15/22
d.22/15
e.11/4
==> in case of power questions, we use the inverse numbers once we see 'together and alone'. In other words, since y and z worked together and took t hours, (1/15)=(1/18)=1/t, (6+5)/90=1/t, t=90/11 therefore 12:(90/11)=2:(15/11). multiplying 11 to both sides gives us 22:15, therefore D is the answer.

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gouthami naidu Newbie | Next Rank: 10 Posts Default Avatar
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Post Sun Feb 21, 2016 9:49 pm
Two important things in time and work :

if X completes work is x hrs, X RATE of work = 1/x
Y completes work is y hrs,Y RATE of work= 1/y

and they work together

1/x + 1/y = 1/h will give the total 'TIME' taken to complete the work (usually days)

where as 'h' gives total 'RATE' of combined x and y

that's why the flip happens

maxmayr93 Newbie | Next Rank: 10 Posts
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Posted:
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Post Mon Sep 25, 2017 6:43 am
Hello all,

so far I have only understood the answer from "GMATinsight" which makes totally sense.

I have a different approach and want to know why I can't proceed as followed:

the medium time of Printer y and z to finish the certain Job is 16,5 hours.

I divided the 16,5 by two because I assume that with the average time the two Printers will print twice as many as one Printer. The result should be proportioned to the result of the time Printer x Needs for this certain printing Job.

Thanks for your help in advance and Kind regards,
Max

Post Mon Sep 25, 2017 9:49 am
maxmayr93 wrote:
Hello all,

so far I have only understood the answer from "GMATinsight" which makes totally sense.

I have a different approach and want to know why I can't proceed as followed:

the medium time of Printer y and z to finish the certain Job is 16,5 hours.

I divided the 16,5 by two because I assume that with the average time the two Printers will print twice as many as one Printer. The result should be proportioned to the result of the time Printer x Needs for this certain printing Job.

Thanks for your help in advance and Kind regards,
Max
This is where intuition fails.

The time it takes for both Y and Z working together is as follows:

1 = T(1/18 + 1/15) . Solving for T yields 90/11 hours, or 8 2/11 hours.

So each of them working 8 2/11 hours will finish the job. This differs from a simple average you mention above of 8 1/4 hours.

2/11 = 8/44 versus 1/4 = 11/44 > your method has them working 3/44 longer as necessary.

The 15 and 18 hours can be viewed as a form of rate, which is a ratio, and you have to be very careful averaging ratios and how you interpret them

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