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messed up in my DS section

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messed up in my DS section [#permalink] New post 14 Sep 2009, 11:06
guys , im all messed up in the DS department right now :oops: ... any help on how to improve on DS please ... :roll:
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Re: messed up in my DS section [#permalink] New post 14 Sep 2009, 11:32
ronniebassist wrote:
guys , im all messed up in the DS department right now :oops: ... any help on how to improve on DS please ... :roll:



That's a bit vague, but I'll try to help.

Are you having trouble with the problem solving aspect of the question (the quant side) or with the approach to aspect of the question (the strategy)?

In my sig there is a link to the strategy thread. There is a fairly solid DS strategy listed there. That being said, strategies assume you have a solid grounding in the material.

Let me know where you need help and lets move forward.
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Re: messed up in my DS section [#permalink] New post 14 Sep 2009, 12:06
mohater wrote:
ronniebassist wrote:
guys , im all messed up in the DS department right now :oops: ... any help on how to improve on DS please ... :roll:



That's a bit vague, but I'll try to help.

Are you having trouble with the problem solving aspect of the question (the quant side) or with the approach to aspect of the question (the strategy)?

In my sig there is a link to the strategy thread. There is a fairly solid DS strategy listed there. That being said, strategies assume you have a solid grounding in the material.

Let me know where you need help and lets move forward.



there was this certain question :

What is the volume of a certain rectangular cuboid ?
(1) Two adjacent faces of the solid have areas 30 and 48 , respectively .
(2) Each of the two opposite faces of the cuboid has area 60 .

It might sound ultra dumb , but I couldnt even get started with this . What kind of strategy should I have adopted here ? , and is there a different approach for DS problems from different areas i.e algebra , geometry etc.
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Re: messed up in my DS section [#permalink] New post 14 Sep 2009, 12:42
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ronniebassist wrote:


there was this certain question :

What is the volume of a certain rectangular cuboid ?
(1) Two adjacent faces of the solid have areas 30 and 48 , respectively .
(2) Each of the two opposite faces of the cuboid has area 60 .

It might sound ultra dumb , but I couldnt even get started with this . What kind of strategy should I have adopted here ? , and is there a different approach for DS problems from different areas i.e algebra , geometry etc.


Sounds like you just need to become comfortable with what DS questions are asking.

A DS question is asking "Do I have enough to solve this problem". The answer can be yes or no and be sufficient, but the key is it MUST be definitive either way.

Spend some time becoming familiar with DS questions and how to approach them.

As for your specific question:

This is a geometry problem, trying to validate the minimum data needed to solve for the volume.

Since it is a rectangular cuboid, you have a minimum of two different leg lengths, and potentially three.

Volume is simply length x width x height right? So we need to be able to solve for three distinct variables, and it's possible two of them are the same.

Let's start with option two. Option two opposite faces have area 60. If we factor out 60 for the possible retangble dimension combinations we find: are 60x1 30x2 20x3 15x4 12x5 10x6

We also are not given any information regarding the third leg (that could be the same or different with one of the other two), as opposites side of a rectangular cube are exactly the same.

Options two is not sufficient on its own.

One tells us information about two adjacent sides. This means one leg between them must be shared, and you also come out with the third leg, so this one looks promising (but don't jump to conclusions just yet).

Factoring out 30 we get 30x1 15x2 10x3 5x6
Factoring out 48 we get 48x1 24x2 16x3 12x4 8x6

We have quite a few that share a common length. Namely the factors w/ 1, 2, 3, 6, with different volumes accordingly.

Thus two on its own is not sufficient.

If we think about how a cube is shaped, we can go through to see if these combination actually work by combining option 1 and option 2.

We know the area of each rectangle. 60, 30 and 48.

Start matching up each one until you get a set that works. If you have more than one combination that works (and yield different volumes) then you know both together are NOT sufficient. If no matter what you only have one result, together they ARE sufficient.

I would solve it right now, but my brain is entirely dead. I will try to remember to come back to this one later.
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Re: messed up in my DS section [#permalink] New post 14 Sep 2009, 13:18
thanks a ton for the reply mate :) . The solution where you have mentioned that one side is share between the two faces struck my mind , but i wasnt able to build up on it . I think i'll get familiar with these DS questions after practicing a few from each specific quants topic .
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Re: messed up in my DS section [#permalink] New post 14 Sep 2009, 13:51
I messed up in the math above. Once I stop being so tired I'll fix it, but you see the approach to the problem.
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Re: messed up in my DS section [#permalink] New post 14 Sep 2009, 17:03
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Where is this question from?

ronniebassist wrote:


there was this certain question :

What is the volume of a certain rectangular cuboid ?
(1) Two adjacent faces of the solid have areas 30 and 48 , respectively .
(2) Each of the two opposite faces of the cuboid has area 60 .

It might sound ultra dumb , but I couldnt even get started with this . What kind of strategy should I have adopted here ? , and is there a different approach for DS problems from different areas i.e algebra , geometry etc.

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Re: messed up in my DS section [#permalink] New post 14 Sep 2009, 22:33
this particular question is from OG 12's DS section .

Im finding that from the DS section the question from 110 onwards in OG 12 are really a hard nut to crack . From 1-100 i hardly got 6-7 of em' wrong , but the final few questions are as tough as hell :twisted: , any particular approach to tackle the tough problems , cos most of em had solutions that I couldnt have figured out even if i had spent an hour on each one of them :roll:
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Re: messed up in my DS section [#permalink] New post 15 Sep 2009, 01:06
Remember, most people say that the questions in the OG book are in ascending difficulty order, thus the later ones tend to be much harder than the early ones.

The key with DS problems is break it down as much as possible. We see with your example it's a question about the three dimensions of a shape, and you're asked to solve for the volume (the shape is not uniform in shape and size). This mean we possibly have three distinct values and your goal becomes "Do I have the values for the dimensions?"

When you break down DS problems, you get to the root of the question, which is something that you can actually solve for. The test writers intentionally make the question convoluted and confusing. Your goal is to break that up and figure out what the question is really asking.

Browse the DS forum and try to solve the problems people post. If you don't understand, read the explanations provided by others.
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Re: messed up in my DS section [#permalink] New post 15 Sep 2009, 05:54
My approach above is known as the "brute force" approach. It's where one tries to solve the problem in its entirety.

This is a bad idea on the GMAT (due to time constraints). Trying to solve this when I was dead tired last night was also a bad idea. bb helped out and provided a better approach.

bb wrote:
In a cuboid, Volume is a*b*c where abc are sides of a cuboid.
From (1), we have ab = 30 and bc = 48
Thus a = \frac{30}{b}, c = \frac{48}{b}

so, a*b*c = \frac{1440}{b}. Not sufficient.

S2 is not sufficient.

S1 is not sufficient as you have even less information (information about one of three pairs of rectangles)

However, combining the two statements, we get ab*bc*ca = 30*48*60
or a^2b^2c^2 = 30*48*60 or abc = \sqrt{30*48*60} Thus sufficient.

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Re: messed up in my DS section [#permalink] New post 15 Sep 2009, 13:00
mohater wrote:
My approach above is known as the "brute force" approach. It's where one tries to solve the problem in its entirety.

This is a bad idea on the GMAT (due to time constraints). Trying to solve this when I was dead tired last night was also a bad idea. bb helped out and provided a better approach.

bb wrote:
In a cuboid, Volume is a*b*c where abc are sides of a cuboid.
From (1), we have ab = 30 and bc = 48
Thus a = \frac{30}{b}, c = \frac{48}{b}

so, a*b*c = \frac{1440}{b}. Not sufficient.

S2 is not sufficient.

S1 is not sufficient as you have even less information (information about one of three pairs of rectangles)

However, combining the two statements, we get ab*bc*ca = 30*48*60
or a^2b^2c^2 = 30*48*60 or abc = \sqrt{30*48*60} Thus sufficient.



thanks for the solution , from now on , i shall try to shred a problem to its mere basics , and then go forward with the DS part of it :P
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Re: messed up in my DS section   [#permalink] 15 Sep 2009, 13:00
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