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Updated on: 16 Jun 2022, 19:17
Originally posted by GmatKnightTutor on 17 Nov 2020, 19:03.
Last edited by GmatKnightTutor on 16 Jun 2022, 19:17, edited 12 times in total.
Last edited by GmatKnightTutor on 16 Jun 2022, 19:17, edited 12 times in total.
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Updated on: 10 Apr 2022, 08:49
Originally posted by IanStewart on 18 Nov 2020, 11:22.
Last edited by IanStewart on 10 Apr 2022, 08:49, edited 1 time in total.
Last edited by IanStewart on 10 Apr 2022, 08:49, edited 1 time in total.
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Edit (April 2021) -- I originally wrote the below in reply to a 'tip' suggesting a count-equations-count-unknowns DS strategy. I'm not sure what happened to the original post I was replying to. gmatclub has moved posts around to create this compilation thread, and my post has somehow found its way here. Since I don't ever delete things I've written, or edit posts to fix mistakes more serious than typos (I don't ever try to hide any mistakes I make, and I only ever add notes like this one clearly labeled as later edits), I'm adding this comment to explain why this post is here. I did not post it to this thread.
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It's true if you invent three random linear equations in three unknowns, you'll probably be able to solve. But GMAT questions are not random. They're constructed to test if you know how math works -- if you can identify when you can solve, and when you cannot. They test the exceptions just as often as the "rules". That's why any DS strategy based on counting equations and counting unknowns is essentially no better than random guessing.
That's empirically verifiable. If you try using "count equations, count unknowns" as a strategy on a large sample of official DS questions, at least on medium and hard questions, you only get the right answer about 1/3 of the time. On DS, when you can usually rule out a couple of answers by logic (because one statement alone is clearly insufficient, say), a strategy that answers 1/3 of your questions correctly is effectively no better than random guessing. The strategy does "work" about 2/3 of the time on easy-level questions, but getting only 2/3 of the easy questions right on the GMAT is not an acceptable hit rate for anyone aiming for a score above Q30 or so.
The best practice is to actually solve in situations like this. You'll discover very quickly whether you can solve or not. Especially when you have 3 unknowns, there are no simple rules (we have a fairly simple rule for 2 linear equations and 2 unknowns, but things are much more complicated with 3 unknowns).
_________________
It's true if you invent three random linear equations in three unknowns, you'll probably be able to solve. But GMAT questions are not random. They're constructed to test if you know how math works -- if you can identify when you can solve, and when you cannot. They test the exceptions just as often as the "rules". That's why any DS strategy based on counting equations and counting unknowns is essentially no better than random guessing.
That's empirically verifiable. If you try using "count equations, count unknowns" as a strategy on a large sample of official DS questions, at least on medium and hard questions, you only get the right answer about 1/3 of the time. On DS, when you can usually rule out a couple of answers by logic (because one statement alone is clearly insufficient, say), a strategy that answers 1/3 of your questions correctly is effectively no better than random guessing. The strategy does "work" about 2/3 of the time on easy-level questions, but getting only 2/3 of the easy questions right on the GMAT is not an acceptable hit rate for anyone aiming for a score above Q30 or so.
The best practice is to actually solve in situations like this. You'll discover very quickly whether you can solve or not. Especially when you have 3 unknowns, there are no simple rules (we have a fairly simple rule for 2 linear equations and 2 unknowns, but things are much more complicated with 3 unknowns).
Updated on: 07 Apr 2022, 20:55
Originally posted by GmatKnightTutor on 21 Jan 2021, 18:35.
Last edited by GmatKnightTutor on 07 Apr 2022, 20:55, edited 2 times in total.
Last edited by GmatKnightTutor on 07 Apr 2022, 20:55, edited 2 times in total.
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Looking at DS statements. Sometimes it may be good to just take a moment and simply look at them.
B + C > 0
Suppose you saw the above inequality. What is one thing you could definitely take away from that? Some piece of information you could write about?
Or are you someone who immediately writes B > - C and leaves it there? Hoping something later on will immediately make sense with your re-arrangement?
To be clear, there is nothing wrong with rewriting an inequality. It can be beneficial sometimes. But it can also be beneficial to leave a statement as it is (or go back to the original form), so that conceptual information can be more readily seen.
B + C > 0
This inequality tells you that B + C is greater than zero (fairly obviously)... but it also tells you that AT LEAST one variable out of those two, b and c, is positive as well (since the other could be negative).
8 + (-5) > 0
8 + 5 > 0
I know. The conceptual takeaway is not much more profound than that. But the point of all this is to simply show that taking a moment to just look at a statement and think about what it could mean can be better than immediately diving in and doing some algebra stuff.
B + C > 0
Suppose you saw the above inequality. What is one thing you could definitely take away from that? Some piece of information you could write about?
Or are you someone who immediately writes B > - C and leaves it there? Hoping something later on will immediately make sense with your re-arrangement?
To be clear, there is nothing wrong with rewriting an inequality. It can be beneficial sometimes. But it can also be beneficial to leave a statement as it is (or go back to the original form), so that conceptual information can be more readily seen.
B + C > 0
This inequality tells you that B + C is greater than zero (fairly obviously)... but it also tells you that AT LEAST one variable out of those two, b and c, is positive as well (since the other could be negative).
8 + (-5) > 0
8 + 5 > 0
I know. The conceptual takeaway is not much more profound than that. But the point of all this is to simply show that taking a moment to just look at a statement and think about what it could mean can be better than immediately diving in and doing some algebra stuff.
