If you only knew then what you know now... Fave GMAT Tips

Question

Hello, everyone:

As I was discussing with a group of Veritas instructors yesterday in one of our instructor development sessions, there are a lot of GMAT strategies that I've learned over the years that I now wonder how I ever took the GMAT without! In discussing the curriculum for our new strategy-heavy Essentials Course, we all agreed that there are quite a few strategies that make taking the GMAT infinitely easier that we wish someone would have told us from day one.

So I thought it would be a good thread if people want to chime in with the one GMAT strategy/tip/concept that they've learned recently that they wish they had known all along. Who's with me?

Probably my personal favorite is Prime Factorization. The more I break numbers down into their primes, the more useful I find that whole thought process.

For example, on a problem like:

How many unique factors does 36 have?

Instead of trying to list them all, you can break 36 down to:

2*2*3*3

And then build up from there with combinations of those numbers:

2
3
2*2
2*3
3*3
2*2*3
2*3*3
2*2*3*3
And, of course, 1, which is a factor of all numbers. So 36 has 9 factors, and there's a systematic way to arrive at that. And using the Prime Factorization technique has taught my how to break down a lot of questions that have "factor" or "divisible" in them somewhere - it's a guiding principle that so often works.

Does anyone else have a favorite "wow, I wish I had known that all along" GMAT strategy? I'm interested to hear yours...

whiplash2411 · Answer

When you are asked to, for example find the value of 1000/1001 and the answer choices are 0.998, 0.997 and 0.999 - you really are left with no choice but to do long division. But there is a simple trick for this:

When you are asked to find the value of a/a+b where b is a small number, you can use the approximation that the fraction is equal to a - b. Quite a neat trick eh?

As for strategies, eliminating the scope of answers in Verbal questions by usage of relevance and tone really really helps a lot. Especially in assumption questions with causal effect involved. If you have a causal effect, that says X causes Y, you are intrinsically assuming that
1. Y does not cause X
2. Z does not cause Y

Usage of these two simple cases will help you eliminate a lot of choices or just arrive at the right one through prephrasing!

drisss · Answer

I just like the idea... thanks for those tips, I'll do the same if I have one

zisis · Answer

or you can always use  (x+1)(y+1)(z+1)....  where x and y refer to the powers of each unique prime factor:
eg
 36=2^2 3^2  therefore 36 has  (2+1)(2+1)=3*3=9  factors

spyguy · Answer

these are really helpful. Thanks guys!

KarishmaB · Answer

Before I took my GMAT some years back, I had looked at only some OG questions in a span of 15 days! So Brian, you can imagine that after I went through our books, there were many 'Oh, I wish I knew this strategy before' moments!

My favorite is "The most important aspect of solving most CR questions is finding the correct conclusion." 
I used to read the stimulus, read the question stem and find the appropriate answer. I am sure I made some terrible mistakes in my Verbal section at that time. Now, each and every CR question makes so much sense. It's unbelievable...

VeritasPrepBrian · Answer

Oh, absolutely right, Karishma - I think we've talked about it but I often tell my students to do a CR drill in which they:

1) Read the question stem first
2) If it's a Strengthen/Weaken/Explain question write down the conclusion (or keywords) and stop there, moving on to the next question.
2a) Email me the conclusions that they've identified 
3) Go back and finish the problems

I can predict with >90% accuracy which questions they'll get right or wrong just based on the conclusions they've identified. If they're on the conclusion, they're almost always right; if they've missed it, they almost never are.

One thing I want to add to my original strategy on here - zisis is right that there's a trick for the number of factors in a given number (using the exponents, adding one to each, and multiplying), but I definitely want to add a little caution to focusing too much on those tricks. The GMAT is pretty slick...the authors have a way of knowing which tricks have become popular (in fact, they've been known to browse these forums to see how people are doing on harder questions - hey, guys!) and then write questions that circumvent the trick. Accordingly, while it can certainly save time to know the tricks, I think it's significantly more important to know the concepts that underlie them (e.g. Prime Factorization). The Veritas Prep blog has a post on that today:

https://www.veritasprep.com/blog/2010/12/gmat-tip-of-the-week-gmat-math-the-gob-bluth-way/

MisterEko · Answer

I used to completely struggle with Data Sufficiency, but after I had read the Veritas Prep DS book, I started getting 80-90% of them right. The simplest trick of them all is to read the stem, and then immediately start looking at the easier of two pieces of information, for example, if you get these two options:

1) x+y-6+2xy= 23

and

2) x=6

I used to go to 1) and analyze it. Now, I just pick the easier one to start, which in this case would be statement 2). When you are able to easily eliminate 1 of the statements, it gives you a decent confidence boost because you narrowed your choices down by 40-60%.

VeritasPrepBrian · Answer

Great advice, MisterEko - and not only does that boost confidence but it also helps you to get a better feel for what kinds of information you might need in the "trickier" statement. In your example, x = 6 for statement 2 may get you to recognize that statement 1 only works if x is positive or something like that. Because you're actively starting to work with the problem, you're getting your mind around how to solve it, and that can be extremely helpful.

While we're on that - here's another tip about "the easier statement":

When a statement is clearly insufficient on its own, you MUST make a decision about whether:

-You need that information to make the other statement work (you're shooting for C)

OR

-They're trying to make you think you need that statement, but the other statement actually already tells you that information (so it's A or B)

This is a strategy we call "Why Are You Here?" - you need to determine why the "weak" individual statement was provided. For example, consider the question:

How many integers x exist such that a < x < b?

1) b - a = 6

2) a and b are nonintegers

Statement 2 is clearly insufficient - it tells us nothing about x and the range of a and b is infinite. So, here, you can infer that "nonintegers" is a decision point for you - do you need to be told that a and b are nonintegers, or is that something that's embedded within statement 1 or that's just unimportant?

Because you know that you need to make this decision, you should test statement 1 with both integers and nonintegers:

Integers: 7 - 1 = 6, values of x are 2, 3, 4, 5, and 6 ---> answer is 5

Nonintegers: 7.5 - 1.5 = 6, values of x are 2, 3, 4, 5, 6, and 7 ---> answer is 6

Now we know that statement 1 is insufficient on its own (we got two answers) and we knew that statement 2 was no good, so the answer is C. And by using the weaker statement, statement 2, as a decision point, we could efficiently derive all of that.

MisterEko · Answer

Isn't it nice when we discover ways to beat the GMAT with its own weapons? 8-)

spyguy · Answer

This keeps getting better and better. I hope more people happen to stumble in this forum like I did. Thanks guys.

thegmatguru · Answer

I have a pretty extensive discussion of how to find all the factors of any number on my website, 790gmat. Just look for "Free Prime Factorization Problems" on the right side of the homepage. In particular, you should read page 1 and attempt questions 15 and 16.

The GMAT Guru

The GMAT Guru

drisss · Answer

Hi Mr. The Gmat Guru,

I think that your GMAT score is pretty appealing to us... I will have a look at your website. Who knows, maybe that could bring me some inspiration for d-day.

tfincham86 · Answer

This is a great tip! I sometimes find myself bogged down on Stmt 1, taking up too much time and then making me nervous that I'm not doing something right.

thegmatguru · Answer

What is more interesting about the example above and something that most prep companies don't tell you is that IF YOU CORRECTLY GET ONLY ONE SOLUTION TO ONE OF THE STATEMENTS, THAT SOLUTION WILL AUTOMATICALLY BE A SOLUTION TO THE OTHER STATEMENT.

For example:

What is the value of x?

1) x is a prime factor of 221
2) 2x = 34

Clearly, the second statement is easier to evaluate (sufficient..only answer is 17)

If you know this, the first statement is much easier to evaluate because you KNOW that x=17 is a solution and you can immediately divide 221 by 17 to see what other factors it may have. (17*13 = 221)

Note that knowing the answer to one statement does not tell you how many solutions the other has, so it won't tell you directly "insufficient" or "sufficient", but it gives you at least one solution to start with.

spyguy · Answer

GMAT Guru...forgive me if you have already posted it but can you share your website?

Thanks.

thegmatguru · Answer

Just google "The GMAT Guru" and my website 790gmat should come up. GMATclub won't allow new users to post urls directly, otherwise I would just post it.

M23A · Answer

Thanks for all the great tips.

https://gmatclub.com/static/posted_from.png     Posted from    GMAT ToolKit

marcovg4 · Answer

Hi GMAT Guru, what is the answer for this SC question? Thanks

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