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og math # 130

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chirpy Newbie | Next Rank: 10 Posts Default Avatar
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Post Fri Jul 26, 2013 12:15 am
C is the correct answer.

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shubhamp Newbie | Next Rank: 10 Posts
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Post Thu Apr 24, 2014 8:35 am
12/15*18/(15+18)
= 12*33/15*18
=22/15

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daboo343 Newbie | Next Rank: 10 Posts
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Post Fri Feb 14, 2014 11:58 pm
22/15
(D)

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shubhamp Newbie | Next Rank: 10 Posts
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Post Thu Apr 24, 2014 8:35 am
12/15*18/(15+18)
= 12*33/15*18
=22/15

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daboo343 Newbie | Next Rank: 10 Posts
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Post Fri Feb 14, 2014 11:58 pm
22/15
(D)

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GMATinsight Legendary Member
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Post Thu Jul 24, 2014 5:57 am
Quote:
resilient wrote:
working alone, printers x,y, and z can do a certain printing job, consistning 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

Let's assume the Total work in terms of "Work units" by assuming it a number which is a Common multiple (not essentially the LCM) of 12, 15 and 18. This exercise is specially beneficial in order to avoid the calculation of fraction.


A common multiple of 12, 15 and 18 = 180 (One can also assume this number as 360, 540 etc. however the least is the best]

Since the 180 units is done by X in 12 Hours, Y in 15 hours and X in 18 hours therefore

- One hour of Printer X : 180/12 = 15 Units.
- One hour of Printer Y : 180/15 = 12 Units.
- One hour of Printer Z : 180/18 = 10 Units.

- One hour of Printer Y and Z together : 12+10 = 22 Units.
Total time taken by Y and Z to finish the work together = 180/22 = 90/11 hours

So the Required ratio is: 12/(90/11) = 22/15

Answer: Option D

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dmv Newbie | Next Rank: 10 Posts
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Post Thu Jul 24, 2014 12:24 am
1/15 +1/18 = 11/90

Y and Z together = 90/11

therefore ratio = 12/1/90/11 = 22/15

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visufun Newbie | Next Rank: 10 Posts
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Post Tue Jul 22, 2014 11:22 pm
Dont jump and solve this problem straight away.

You can straight away eliminate choices a, b and c as those choices dosen't make any sense. ( time for A to complete the job will always be greater than time for B & C working together)

Now we are left with D and E.

pick a comfortable number which divides 12, 15 and 18. I chose 180. time taken for printing 180 units will be,

A - 15
B - 12
C - 10

180/15 by 180/(12 +10) = 22/15

Answer: D

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Joseph_Alexander Senior | Next Rank: 100 Posts Default Avatar
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Post Mon Jul 07, 2014 11:30 pm
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
Hi resilent!

I got the same answer as yours. Noticed that 12, 15 and 18 is the time they take to complete a job and it is not their speed. So assuming that the work is 180, their speed would be 15, 12 and 10. Now if you use 15, 12 and 10, you won't have to flip! Smile

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kshitijhbti Newbie | Next Rank: 10 Posts Default Avatar
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Post Mon Feb 03, 2014 12:07 am
Answer is D.

Time taken by x = 12

Time taken y and z to work together = (18*15)/(18+15) = 270/33 = 90/11

Ratio= x/(y+z)= 12/(90/11) = 12*11/ 90 = 22/15

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sansulee Junior | Next Rank: 30 Posts Default Avatar
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Post Sun Nov 03, 2013 8:38 am
GMATGuruNY wrote:
resilient wrote:
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

qa is d. I dont see why C is wrong. I dont see why the solution flips the combined rate of y and z working together. help stuart?
I think that the easiest approach is to plug in a value for the job in order to determine everyone's respective rates.

Plug in job = 180.
Rate for x = w/t = 180/12 = 15/hour.
Rate for y = w/t = 180/15 = 12/hour.
Rate for z = w/t = 180/18 = 10/hour.
Combined rate of y+z = 12+10 = 22/hour.
Time for y+z = w/r = 180/22 = 90/11.
Ratio of (time x):(time y+z) = 12/(90/11) = 22/15.

The correct answer is D.
Could you explain why the answer is D?

Aren't we supposed to take the ratio of the "rates" of these individual entities? (considering x as one and the combo of y-z as another entity)?

In that case, shouldn't the calculation go as:

1/12 / 33/15*18 = 15*18 / 33*12 ; eventually yielding 15/22 ?

Please correct me if I am wrong, so that I do not repeat the mistake.

Thanks!

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sanju09 GMAT Instructor
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Post Thu Oct 24, 2013 5:07 am
This thread would prove out to be an EPIC on BTG.

http://www.beatthegmat.com/og-math-130-t9128-105.html#699281

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yogesh_yadav Newbie | Next Rank: 10 Posts Default Avatar
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Post Wed Oct 23, 2013 4:32 am
22/15

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prats14 Newbie | Next Rank: 10 Posts Default Avatar
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Post Tue Sep 24, 2013 9:37 am
22/15

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leekaru14 Newbie | Next Rank: 10 Posts
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Post Wed Sep 18, 2013 12:21 am
The answer is choice D.
I divide the workload into 180 widgets.
In one hour:
- Printer x can finish: 180/12 = 15 widgets.
- Printer y can finish: 180/15 = 12 widgets.
- Printer z can finish: 180/18 = 10 widgets.
So in one hour y and z can finish: 22 widgets. Therefore it takes them 190/22 = 90/11 hours to finish the work.
So the ratio is: 12/(90/11) = 22/15

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