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Time management when testing cases in DS

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Time management when testing cases in DS [#permalink]

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New post 16 Jun 2015, 05:18
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Hi,

Does anyone have any suggestions on speeding up testing cases for DS questions? I take much longer on DS, especially 700+ questions. My accuracy isn't a problem but it takes me a long time to think of cases both ways for each statement.

Here's an example of the kind of question that's taking me 3 minutes plus: (example http://gmatclub.com/forum/if-y-is-not-equal-to-0-and-y-is-not-equal-to-1-which-is-134494.html).

At the moment I'll try to prove 'yes' and 'no' for each prompt statement, before finding cases to test 'C' if necessary. I understand that the 'C' cases can sometimes just involve recycling cases from either statement, but I'm normally 2 minutes in even by the time I'm thinking about C!

How do other people organise their DS approach to cut down on the time involved?

Thanks

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Re: Time management when testing cases in DS [#permalink]

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New post 16 Jun 2015, 05:30
ElCorazon wrote:
Hi,

Does anyone have any suggestions on speeding up testing cases for DS questions? I take much longer on DS, especially 700+ questions. My accuracy isn't a problem but it takes me a long time to think of cases both ways for each statement.

Here's an example of the kind of question that's taking me 3 minutes plus: (example http://gmatclub.com/forum/if-y-is-not-equal-to-0-and-y-is-not-equal-to-1-which-is-134494.html).

At the moment I'll try to prove 'yes' and 'no' for each prompt statement, before finding cases to test 'C' if necessary. I understand that the 'C' cases can sometimes just involve recycling cases from either statement, but I'm normally 2 minutes in even by the time I'm thinking about C!

How do other people organise their DS approach to cut down on the time involved?

Thanks


IMO, for most of the DS questions, you can solve by number picking by the following scenarios:

Negative Numbers
Positive Numbers
Fractions.

I usually go with the following set: ,-100-2,-1.5,-1,-0.5,-0.25,0,0.25,0.5,1,2,100

The DS prompt will prohibit some of the above values and will thus narrow your range. You just have to be smart about your selection.

FYI, +/-100 values are exampels of 'very large values' (usually helpful in questions related to sets/medians/means etc).

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Re: Time management when testing cases in DS [#permalink]

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New post 16 Jun 2015, 07:48
I definitely need to be a bit smarter about choosing from the set list (negative, positives, fractions, 0 etc). But I find it difficult to quickly identify the right values to test when there are two variables. For instance, in the question I link to above, both statements allow x and y to take on positive, fractional or negative values; that's 9 combinations of number type to choose from for x and y!

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Re: Time management when testing cases in DS [#permalink]

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A general outline of DS scenario-generation strategy:

• if you think a statement is sufficient, you should try to prove it, since that's more reliable (and usually faster) than generating scenarios

• if you do not think a statement is sufficient, or you try to prove it is and cannot, then generate scenarios

• first generate the simplest possible scenario to get one answer to the question as quickly as possible

• then, with that answer in mind, step back from the question and ask "what would I need to do to get a different answer to the question?" What you might need to do depends entirely on the math in the question, and the better you understand the mathematical concepts on the GMAT, the faster you'll be able to do this. In, say, an even/odd question, if you tested some even scenario first, you'd then normally test some odd scenario. In an inequality question, you'd most often test a simple positive scenario first, then a negative scenario.

• if you can get two different answers, you've proven your info is not sufficient. If you keep getting the same answer to the question even after trying to get different answers, the information is sufficient unless you missed some exceptional case. So you cannot necessarily be sure, but you should 'guess' it's sufficient, because you'll be right most of the time.

• do not just plug in some arbitrary list of numbers into every question unless you don't understand the question well at all. For one thing, you'll get a lot of questions wrong that way, and for another, you'll waste a lot of time. So for example, many test takers just automatically plug negative fractions into inequality questions, which can be a time-consuming thing to do. Negative fractions are irrelevant in most inequality questions, and if you can recognize when they might matter, and when they don't, you can save a lot of time.

We can use a scenario-generation strategy on the question you linked to. I'm going to change the question very slightly, because the stem contained what I'm sure is a typo (it said y is not 1, but it surely means to say y is not -1, so that the denominator "y+1" is nonzero).

sarb wrote:
If y is not equal to 0 and y is not equal to -1, which is greater, x/y or x/(y+1)

(1) x is not equal to 0
(2) x > y


At first glance, S1 seems to tell us almost nothing. So I'd look at S2 first. I might do some algebra here, but let's use this statement to illustrate scenario generation. And let's look at S2 using examples where x is nonzero (i.e. using S1 too), because if S2 is not sufficient in that case, the answer must be E.

I'd first generate the simplest possible scenario using the information that x > y, just to get any answer to the question very quickly. If we let x = 2 and y = 1, we find that x/y is larger.

Now we want to know if we can also get a different answer to the question - so we want to know if, when x > y, this inequality can be true:

\(\frac{x}{y} < \frac{x}{y+1}\)

I'd first ask: what is different on both sides of this inequality? It's the denominators that differ, so that's what we should probably focus on. I might just plug in a simple value for x, like x=2, so we want to try to make this true when y < x, so when y < 2:

\(\frac{2}{y} < \frac{2}{y+1}\)

Now inequality questions on the GMAT are most often testing positives and negatives. And the easiest way to make an inequality true is to make one side positive and the other negative - positives are always bigger than negatives. So here, I'd ask "can I make the left side negative and the right side positive"? And we can, because we can make the denominator negative on the left side, and positive on the right. So if we let, say, x=2 and y= -1/2, then both statements are true and x/(y+1) is larger than x/y. Since we also found that x/y can be larger, the answer is E.
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Re: Time management when testing cases in DS [#permalink]

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New post 16 Jun 2015, 15:05
Ian, I couldnt agree more. When I mentioned the list of values above, I did mention that one can not start plugging in the values without spending the first 5-10 seconds making sure that one understands what is getting asked and what is given. DS questions are more about understanding what tools do you actually have based on the prompts.

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Re: Time management when testing cases in DS [#permalink]

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New post 16 Jun 2015, 21:52
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ElCorazon wrote:
Hi,

Does anyone have any suggestions on speeding up testing cases for DS questions? I take much longer on DS, especially 700+ questions. My accuracy isn't a problem but it takes me a long time to think of cases both ways for each statement.

Here's an example of the kind of question that's taking me 3 minutes plus: (example http://gmatclub.com/forum/if-y-is-not-equal-to-0-and-y-is-not-equal-to-1-which-is-134494.html).

At the moment I'll try to prove 'yes' and 'no' for each prompt statement, before finding cases to test 'C' if necessary. I understand that the 'C' cases can sometimes just involve recycling cases from either statement, but I'm normally 2 minutes in even by the time I'm thinking about C!

How do other people organise their DS approach to cut down on the time involved?

Thanks


A point to note is that you shouldn't rely on number plugging completely. There will be one obvious answer to the question -Yes or No. You need to spend some time to find a case for the other one.
Start with logic, plug in some numbers to reassure yourself, then try to find the opposite case.

Here is what I mean:

If y is not equal to 0 and y is not equal to 1, which is greater, x/y or x/(y+1)

(1) x is not equal to 0
(2) x > y

Question: Is x/y > x/(y+1)
The obvious answer is 'Yes' because we usually think in terms of positive numbers. Using logic, we would say that when you divide a number by a larger number (y+1) it will obviously become smaller. So x/y must be greater than x/(y+1).

Now think of the less obvious cases:
If x = 0, both are equal.
So what happens when x is negative? x/y will have larger absolute value but will be more negative so will be a smaller number.
-2/2 < -2/3

You see that stmnt 1 is not sufficient so check statement 2. Check for negative value of x since we know that when both x and y are positive and greater than 1, the inequality holds:
Using stmnt 2, if x is negative, y will be a smaller negative number say x = -2, y = -3
-2/-3 < -2/-2 so x/y is less than x/(y+1) in this case. Hence inequality doesn't hold.

Hence answer will be (E)

Basically, focus on Positive values vs 0 vs negative values and fractions vs integers.
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Re: Time management when testing cases in DS [#permalink]

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