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First plug in choice

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First plug in choice

by MSF_Jon » Sat May 09, 2009 9:28 pm
Hello everyone first let me thank you all for this resource. It is invaluable to my preparation.

Second, I have a question regarding where to start when using the answer choices to "plug in" in order to find the correct answer. I am currently taking a prep course at a local college in which the instructor says that starting with C will allow you to quickly answer the question or allow you to eliminate three wrong choices for being too large or small. Thus leaving you with either a 50/50 guess for a quick/time is of the essence situation, or by plugging in one of the two remaining you will find that it is either correct or the only other remaining answer is correct. I can definitely see the logic in this.

Although wouldn't an alternative, albeit more aggressive, method be to start with B or D. If it is correct then great, obviously this is the same as choosing C first. But if for example you choose D and it is too small then you know the answer is E, alternatively if it is too large then plugging in B will be required to find the correct answer.

Of course this goes with a problem that you cannot eliminate any outliers and one must be cognizant of the ascending or descending order of the answers. Maybe I'm just spinning my wheels here but it seems like a minor gamble for a potentially decent time reward.

Any opinions on this?
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by DanaJ » Sun May 10, 2009 12:21 am
All I know is that Kaplan recommended the same method of plugging: using the middle value. I personally think it's safer to go for the middle value, since, as you've pointed out, you start with a 50/50 chance. Besides, one of math teachers used to say that the answer to multiple choice problems is almost always somewhere in the middle: they offer two other options that are close enough to the answer to confuse you and two choice that are "outrageous", so to speak, or too far away from the answer.

I think it's a good strategy to start with the middle. If you start with B, then what if the answer is actually E? You waste three steps. If you start with C, the maximum number of steps that you use will be two. It's risky...

Plugging in numbers is not my favorite way of solving stuff. Sometimes it's preferable: it helps you eliminate choice quickly (especially DS) by providing counterexamples to a problem. HOWEVER, when not dealing with a counterexample, use plugging in with caution: you may be picking a set of numbers that, by chance, just fits. Additionally, if there are extensive calculations involved, just plugging the answer choices is again risky, since you could end up with a lot of wasted time.
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by Ian Stewart » Sun May 10, 2009 8:08 am
Each answer choice is correct equally often on the GMAT, so you're just as likely to get the right answer first try if you start with C as if you start with D.

On some (rather few, in fact) questions, when you test an answer and discover it's incorrect, you'll know whether the correct answer is smaller or larger than what you've tested. On those questions, if you are testing answers, it's certainly most efficient to begin with either B or D. When you start with C:

1 time in 5, you'll have the answer first try
4 times in 5, you'll need to test one other choice (and after that second test, you'll know by process of elimination which answer is correct)

If you start with B:

2 times in 5, you'll have the answer first try (because if B is too large, A must be right)
3 times in 5, you'll need to test exactly one other choice - answer choice D. If D is right, you're done; if it's too large, C must be right; if it's too small, E must be right.

That's exactly what Jon said above, and yes, C is not the best choice to begin with in questions of this type.

All of that said, I don't think it's all that helpful to know this in practice:

-questions where plugging in answer choices is a good idea aren't all that common, at least at the harder level of the test;

-questions where, when plugging in answer choices, you'll know if the right answer is smaller or larger than what you've tested are much less common still;

-if you can solve the question from start to finish, rather than by doing everything backwards, you're guaranteed to do the work once only. Each time you test an answer choice, you're normally doing all of that work once, so you might be solving the same problem two or more times by backsolving.

It's a worthwhile fallback technique when you aren't able to see how to do a problem directly, and there's no harm in knowing how to backsolve and practicing it - it is a strategy that is potentially useful on lower level questions, and it has the advantage of removing some of the abstraction from some questions. It's not a technique that is especially helpful on more difficult questions, however; I analyzed the more difficult questions in the Official Guide (11th edition) to see how many could be backsolved, and wrote about the results here if you're interested:

https://gmatix.com/blogging/index.php?x=15

https://gmatix.com/blogging/index.php?x=20
For online GMAT math tutoring, or to buy my higher-level Quant books and problem sets, contact me at ianstewartgmat at gmail.com

ianstewartgmat.com
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