If you’re going to do a great job on Data Sufficiency, then you’ve got to know how to Test Cases. This strategy will help you on countless DS problems.

Try this GMATPrep® problem from the free exams. Give yourself about 2 minutes. Go!

* “On the number line, if the number k is to the left of the number t, is the product kt to the right of t?

“(1) t < 0 “(2) k < 1” If visualizing things helps you wrap your brain around the math (it certainly helps me), sketch out a number line:

k is somewhere to the left of t, but the two actual values could be anything. Both could be positive or both negative, or k could be negative and t positive. One of the two could even be zero.

The question asks whether kt is to the right of t. That is, is the product kt greater than t by itself?

There are a million possibilities for the values of k and t, so this question is what we call a theory question: are there certain characteristics of various numbers that would produce a consistent answer? Common characteristics tested on theory problems include positive, negative, zero, simple fractions, odds, evens, primes—basically, number properties.

“(1) t < 0 This problem appears to be testing positive and negative, since the statement specifies that one of the values must be negative. Test some real numbers, always making sure that t is negative.

Case #1:

Testing Cases involves three consistent steps:

First, choose numbers to test in the problem

Second, make sure that you have selected a valid case. All of the givens must be true using your selected numbers.

Third, answer the question.

In this case, the answer is Yes. Now, your next strategy comes into play: try to prove the statement insufficient.

How? Ask yourself what numbers you could try that would give you the opposite answer. The first time, you got a Yes. Can you get a No?

Case #2:

Careful: this is where you might make a mistake. In trying to find the opposite case, you might try a mix of numbers that is invalid. Always make sure that you have a valid case before you actually try to answer the question. Discard case 2.

Case #3:

Hmm. We got another Yes answer. What does this mean? If you can’t come up with the opposite answer, see if you can understand why. According to this statement, t is always negative. Since k must be smaller than t, k will also always be negative.

The product kt, then, will be the product of two negative numbers, which is always positive. As a result, kt must always be larger than t, since kt is positive and t is negative.

Okay, statement (1) is sufficient. Cross off answers BCE and check out statement (2):

“(2) k < 1” You know the drill. Test cases again! Case #1:

You’ve got a No answer. Try to find a Yes.

Case #2:

Hmm. I got another No. What needs to happen to make kt > t? Remember what happened when you were testing statement (1): try making them both negative!

In fact, when you’re testing statement (2), see whether any of the cases you already tested for statement (1) are still valid for statement (2). If so, you can save yourself some work. Ideally, the below would be your path for statement (2), not what I first showed above:

“(2) k < 1” Case #1:

Now, try to find your opposite answer: can you get a No?All you have to do is make sure that the case is valid. If so, you’ve already done the math, so you know that the answer is the same (in this case, Yes).

Case #2: Try something I couldn’t try before. k could be positive or even 0…

A Yes and a No add up to an insufficient answer. Eliminate answer (D).

The correct answer is (A).

Guess what? The technique can also work on some Problem Solving problems. Try it out on the following GMATPrep problem, then join me next week to discuss the answer:

* “For which of the following functions f is f(x) = f(1 – x) for all x?

“(A) f(x) = 1 – x

“(B) f(x) = 1 – x²

“(C) f(x) = x² – (1 – x)²

“(D) f(x) = x²(1 – x)²

“(E) 

Key Takeaways: Test Cases on Data Sufficiency

(1) When DS asks you a “theory” question, test cases. Theory questions allow multiple possible scenarios, or cases. Your goal is to see whether the given information provides a consistent answer.

(2) Specifically, try to disprove the statement: if you can find one Yes and one No answer, then you’re done with that statement. You know it’s insufficient. If you keep trying different kinds of numbers but getting the same answer, see whether you can think through the theory to prove to yourself that the statement really does always work. (If you can’t, but the numbers you try keep giving you one consistent answer, just go ahead and assume that the statement is sufficient. If you’ve made a mistake, you can learn from it later.)

* GMATPrep® questions courtesy of the Graduate Management Admissions Council. Usage of this question does not imply endorsement by GMAC.

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The following excerpt comes from Top GMAT Prep Courses, a helpful resource for comparing your GMAT prep options, gathering  in-depth course reviews, and receiving exclusive discounts. Top GMAT Prep Courses had the chance to connect with Ron Purewal, one of Manhattan Prep’s veteran GMAT instructors, to ask questions about the GMAT that we hope all prospective MBA candidates will benefit from. Want more? Head on over to the full article!

What are the most common misconceptions of the GMAT that you notice on a regular basis?

“There are two BIG misconceptions in play here.

The first is “knowledge.” Too many people view this test as a monumental task of memorization. A test of knowing stuff. If you’re new to this exam, it’s understandable that you might think this way. After all, that’s how tests have always worked at school, right? Right. And that’s exactly why the GMAT doesn’t work that way. Think about it for a sec: When it comes to those tests, the tests of knowing stuff, you already have 16 or more years of experience (and grades) under your belt. If the GMAT were yet another one of those tests, it would have no utility. It wouldn’t exist. Instead, the GMAT is precisely the opposite: It’s a test designed to be challenging, and to test skills relevant to business school, while requiring as little concrete knowledge as possible.

If you’re skeptical, go work a few GMAT problems.  Then, when the smoke clears, take an inventory of all the stuff you had to know to solve the problem, as opposed to the thought process itself.  You’ll be surprised by how short the list is, and how elementary the things are.  The challenge isn’t the “what;” it’s the “how.” …Continue reading for the second misconception.
 

How common is it for a student to raise his or her GMAT score 100 points or more, and what is the largest GMAT score increase you’ve personally seen while working at Manhattan Prep?

“We’ve seen such increases from many of our students. I’ve even seen a few increases of more than 300 points, from English learners who made parallel progress on the GMAT and in English itself. I don’t have statistics, but what I can give you is far more important: a list of traits that those successful students have in common.

1) They are flexible and willing to change. They do not cling stubbornly to “preferred” or “textbook” ways of solving problems; instead, they simply collect as many different strategies as possible.

2) They are resilient. When an approach fails, they don’t internalize it as “defeat,” and they don’t keep trying the same things over and over. They just dump the approach that isn’t working, and look for something different. If they come up empty, they simply disengage, guess, and move on.

3) They are balanced. They make time to engage with the GMAT, but they don’t subordinate their entire lives to it. They study three, four, five days a week—not zero, and not seven. They review problems when they’re actually primed to learn; they don’t put in hours just for the sake of putting in hours. If they’re overwhelmed, exhausted, or distressed, they’ll shut the books and hit them another day. In short, they stay sane… Continue reading for more traits of successful students.

Studying for the GMAT? Take our free GMAT practice exam or sign up for a free GMAT trial class running all the time near you, or online. And, be sure to find us on Facebook and Google+,LinkedIn, and follow us on Twitter!

Here’s a fairly straightforward GMATPrep question that I’ve seen many-a-student answer incorrectly. See if you can answer the question without writing anything down:

Of the 3,600 employees of Company X, 1/3 are clerical. If the clerical staff were to be reduced by 1/3, what percent of the total number of the remaining employees would then be clerical?

 

A)  25%

B)  22.2%

C) 20%

D) 12.5%

E)  11.1%

Classic GMAT question. Straightforward. Easy to understand. Simple to equate” 1/3 of 3,600 is 1,200 and 1/3 of that is 400. So we’d only have 800 remaining clerical staff and 800/3600 = 2/9, answer B. And like so(oooooooo) many other GMAT questions, we’d have answered incorrectly.

If have some paper in front of you, try to solve the question again by writing down each step along the way. Maybe even include what each of those numbers that you write down mean in the context of the question. Don’t cheat- see if you answer the question differently when you’re forced to write down more than just a simple computation or two.

…

Here’s what I wrote down first on my scrap paper:
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September is the greatest month of the year. At some point in the not-so-distant future, my AC-unit will be able to finally power off after five straight months of keeping me inside, away from the Texas heat and the West Nile carrying mosquitos that the heat brought with it. But more importantly, September means that football is finally back. So with that in mind, here’s four lessons from the college football season for those of you who need help rationalizing your Saturday afternoon absence from your GMAT study place.

1)  Schedule the Cupcake Sections Early

Oregon hasn’t been spending the last three months preparing to face Arkansas State. And when September 1 rolls around, Oregon would prefer to pull its starters sometime early in the second half. A loss to an early season opponents would definitely hurt their BCS chances, but if the Ducks play half-decent football at the start of the season, they can focus on playing their best once Pac-12 teams start traveling to Eugene in late September.

For you, walking away from the test with a 2 on your AWA or IR section could be a bad thing when it comes time to apply to business school. But running up the score on your AWA won’t help your 200-800 score and you don’t want to exhaust your brain during the first hour of your test. But if you’ve thoroughly prepared for the quant and verbal sections of the test, and have watched some tape (such as our IR recordings or AWA labs) on what you need to do for the two warmup sections, you’ll do just fine early and can focus on playing your best once the quant section shows up on your screen.

2) Focus on One Question at a Time

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Invariably when I ask a student what about their strengths and weaknesses related to the GMAT, his list focuses on topics or question types.

• I struggle with the quant section.
• Sentence correction is my best verbal question type.
• I hate data sufficiency.
• I’m good at rate problems, but I can’t figure out probability.
• Etc.

Now, the ability to generate this sort of inventory is important. You should generally devote more study time to those topics and question types where you are weaker. But along with this topic-based inventory, other aspects of your personality and approach will impact your GMAT experience. Understanding these underlying tendencies in yourself can be invaluable to improving your GMAT performance.

In each of the four cases below consider which statement sounds more like you.

1) To solve a challenging problem
A. Give me a formula. Give me an algorithm. As long as I know an approach I can crank through the math and get the problem done.
B. I like the chance to get creative. Drawing diagrams and recognizing patterns is what I do best.

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When not providing insight into the fascinating world of the GMAT, I enjoy watching detective shows on television.  In many episodes, one of the detectives must delve into the mind of the perpetrator “ actually try to think as the perpetrator does.   In so doing, the seemingly random clues come together (often via a slow motion or black and white flashback scene) leading to an insight that breaks the case.

I am going to advocate taking on this television detective mentality in approaching GMAT problems.  Perhaps there is a further parallel as the mind of the GMAT question writer may seem to be just as scary a place as the mind of a criminal. But the ability to think like a GMAT test writer can provide multiple benefits including enabling you to get more questions right and allowing you to have more confidence in your answers.

So let’s try think about three lessons we can take from our favorite crime dramas and apply to the GMAT.

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When do you leave the house without directions? Or perhaps the more modern reference, is when do you start on your way without plugging in your destination into a GPS device? I expect the answer for most of you is when I know where I am going.

Conversely, when you have no idea when you are going, your solution is most likely not to get in your car (or on your bike or public transit) and just start randomly driving around hoping you run into your desired destination. Ideally, you probably look up where you are going and plan out a route. Alternatively, you might know a nearby destination and start heading there (e.g. I know the bike store is near that place I get coffee). Finally, on occasion you may just head to an area where you expect to find a type of business (e.g. Gas stations are usually close to freeway entrances).

Now, all this talk about directions has been fun, but let’s bring the analogy around to GMAT quant problems. Sometimes when you see a GMAT problem, you may understand what the question is asking and see the path to the solution. In these cases, dive right in. Start driving and you are likely to reach your destination because you know “ or at least have a good sense “ of where you are going. Read more

When I make an error, I get excited. Seriously “ you should be excited when you make errors, too. I know that I’m about to learn something and get better, and that’s definitely worth getting excited!

Errors can come in several different forms: careless errors, content errors, and technique errors. We’re going to discuss something critical today: how to learn from your errors so that you don’t continue to make the same mistakes over and over again. First, let’s define these different error types.
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What’s the optimal way to spend your last 14 days before the real test? There are two levels to this discussion: what and how to review, and building a game plan. We’ll discuss the latter topic in this article. Once you’ve read this article, you may want to check out How To Review.

What is a Game Plan?

In the last two weeks before your test, your focus needs to shift from trying to learn new things to acknowledging that your skills are what they are. They’re not going to change an enormous amount in the last two weeks; you can tweak some things, but now is not the time to change major strategies across an entire question type. Further, it would be a mistake to spend your last two weeks entirely focused on your weaknesses; if you do that, then you won’t be prepared to excel on your strengths.

Your Game Plan will help you to make certain decisions quickly during the test. When is it a good idea to spend an extra 20 or 30 seconds on a problem? When should you decide to make an educated guess? When should you cut yourself off completely, guess immediately, and move on? What should you do if you find yourself ahead or behind on your timing? (We’ll discuss the answers to these questions later in the article.) Read more

by Jonathan McEuen, guest blogger

Jonathan McEueun is a Manhattan GMAT grad who is off to Wharton this fall. We asked him to share his application process with us. What follows is Part 3 of 5 posts in a series about his experiences. You can read Part 2 here.

Don’t Get Lost Before The First Step

The big question of How should I prepare for this test? quickly becomes a set of much more detailed, specific questions:  Do I enroll in a course? Should I buy books and study on my own?  What if I need to take the test multiple times?  All this tends to become a little overwhelming.

I tried to calm down and bring myself back to the first question.  I knew I needed structure and guidance.  I again turned to friends for recommendations.  It was word of mouth that Read more