Let’s talk about GMAT Data Sufficiency.
Specifically, let’s talk about getting GMAT Data Sufficiency (DS) problems wrong. And I don’t mean those problems that you missed because of careless math errors, or because of concepts you hadn’t learned yet. No, I’m talking about the missed DS problems that make you want to bang your head against the wall: How on Earth did I not get that?
There are two reasons you might have this experience: Read more
Recently, we took a look at a story problem dealing with ratios, and I finished up by giving you a second problem to test your skills. How did you do?
If you haven’t already, try the GMATPrep® problem below and then we’ll talk about it. Give yourself about 2 minutes. Go! Read more
How are you with story problems? Most math concepts can be presented in story form on the test and the GMAT test writers do like to get wordy with us. You’ve got a double task: you have to translate the words into math and then you still have to do the math! How can we get through these as efficiently as possible?
Try the GMATPrep® problem below and then we’ll talk about it. Give yourself about 2 minutes. Go!
Recently, a colleague of mine shared this very interesting puzzle published by the New York Times. (Thanks, Ceilidh!)
Go ahead and try it. I’ll wait. After you’ve tried the puzzle, you can read the short article that goes with it.
What did you learn about how humans tend to think? More important, what did you learn about how you think?
That tendency to look for the no, or to try to disprove something, is a trait shared by scientists, devil’s advocates, and great standardized test takers. You can learn to make this your natural reaction, too!
I have now done every last one of the new quant problems in both new books—and there are some really neat ones! I’ve also got some interesting observations for you. (If you haven’t yet read my earlier installments, start here.)
In this installment, I’ll discuss my overall conclusions for quant and I’ll also give you all of the problem numbers for the new problems in both the big OG and the smaller quant-only OG.
What’s new in Quant?
Now that I’ve seen everything, I’ve been able to spot some trends across all of the added and dropped questions. For example, across both The Official Guide for GMAT® Review (aka the big book) and The Official Guide for GMAT® Quant Review (aka quant-only or the quant supplement), Linear Equation problems dropped by a count of 13. This is the differential: new questions minus dropped questions.
That’s a pretty big number; the next closest categories, Inequalities and Rates & Work, dropped by 5 questions each. I’m not convinced that a drop of 5 is at all significant, but I decided that was a safe place to stop the “Hmm, that’s interesting!” count.
Now, a caveat: there are sometimes judgment calls to make in classifying problems. Certain problems cross multiple content areas, so we do our best to pick the topic area that is most essential in solving that problem. But that 13 still stands out.
The biggest jump came from Formulas, with 10 added questions across both sources. This category includes sequences and functions; just straight translation or linear equations would go into those respective categories, not formulas. Positive & Negative questions jumped by 7, weighted average jumped by 6, and coordinate plane jumped by 5.
Given that Linear Equations dropped and Formulas jumped, could it be the case that they are going after somewhat more complex algebra now? That’s certainly possible. I didn’t feel as though the new formula questions were super hard though. It felt more as though they were testing whether you could follow directions. If I give you a weird formula with specific definitions and instructions, can you interpret correctly and manipulate accordingly?
If you think about it, work is a lot more like this than “Oh, here are two linear equations; can you solve for x?” So it makes sense that they would want to emphasize questions of a more practical nature.
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.
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?
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.
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!
You’ve got a No answer. Try to find a Yes.
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”
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
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.
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!
Have you heard of the C-Trap? I’m not going to tell you what it is yet. Try this problem from GMATPrep® first and see whether you can avoid it
* “In a certain year, the difference between Mary’s and Jim’s annual salaries was twice the difference between Mary’s and Kate’s annual salaries. If Mary’s annual salary was the highest of the 3 people, what was the average (arithmetic mean) annual salary of the 3 people that year?
“(1) Jim’s annual salary was $30,000 that year.
“(2) Kate’s annual salary was $40,000 that year.”
I’m going to do something I normally never do at this point in an article: I’m going to tell you the correct answer. I’m not going to type the letter, though, so that your eye won’t inadvertently catch it while you’re still working on the problem. The correct answer is the second of the five data sufficiency answer choices.
How did you do? Did you pick that one? Or did you pick the trap answer, the third one?
Here’s where the C-Trap gets its name: on some questions, using the two statements together will be sufficient to answer the question. The trap is that using just one statement alone will also get you there—so you can’t pick answer (C), which says that neither statement alone works.
In the trickiest C-Traps, the two statements look almost the same (as they do in this problem), and the first one doesn’t work. You’re predisposed, then, to assume that the second statement, which seemingly supplies the “same” kind of information, also won’t work. Therefore, you don’t vet the second statement thoroughly enough before dismissing it—and you’ve just fallen into the trap.
How can you dig yourself out? First of all, just because two statements look similar, don’t assume that they either both work or both don’t. The test writers are really good at setting traps, so assume nothing.
Some Data Sufficiency questions present you with scenarios: stories that could play out in various complicated ways, depending on the statements. How do you get through these with a minimum of time and fuss?
Try the below problem. (Copyright: me! I was inspired by an OG problem; I’ll tell you which one at the end.)
* “During a week-long sale at a car dealership, the most number of cars sold on any one day was 12. If at least 2 cars were sold each day, was the average daily number of cars sold during that week more than 6?
“(1) During that week, the second smallest number of cars sold on any one day was 4.
“(2) During that week, the median number of cars sold was 10.”
First, do you see why I described this as a “scenario” problem? All these different days… and some number of cars sold each day… and then they (I!) toss in average and median… and to top it all off, the problem asks for a range (more than 6). Sigh.
Okay, what do we do with this thing?
Because it’s Data Sufficiency, start by establishing the givens. Because it’s a scenario, Draw It Out.
Let’s see. The “highest” day was 12, but it doesn’t say which day of the week that was. So how can you draw this out?
Neither statement provides information about a specific day of the week, either. Rather, they provide information about the least number of sales and the median number of sales.
The use of median is interesting. How do you normally organize numbers when you’re dealing with median?
Bingo! Try organizing the number of sales from smallest to largest. Draw out 7 slots (one for each day) and add the information given in the question stem:
Now, what about that question? It asks not for the average, but whether the average number of daily sales for the week is more than 6. Does that give you any ideas for an approach to take?
Because it’s a yes/no question, you want to try to “prove” both yes and no for each statement. If you can show that a statement will give you both a yes and a no, then you know that statement is not sufficient. Try this out with statement 1
(1) During that week, the least number of cars sold on any one day was 4.
Draw out a version of the scenario that includes statement (1):
Can you find a way to make the average less than 6? Keep the first day at 2 and make the other days as small as possible:
The sum of the numbers is 34. The average is 34 / 7 = a little smaller than 5.
Can you also make the average greater than 6? Try making all the numbers as big as possible:
(Note: if you’re not sure whether the smallest day could be 4—the wording is a little weird—err on the cautious side and make it 3.)
You may be able to eyeball that and tell it will be greater than 6. If not, calculate: the sum is 67, so the average is just under 10.
Statement (1) is not sufficient because the average might be greater than or less than 6. Cross off answers (A) and (D).
Now, move to statement (2):
(2) During that week, the median number of cars sold was 10.
Again, draw out the scenario (using only the second statement this time!).
Can you make the average less than 6? Test the smallest numbers you can. The three lowest days could each be 2. Then, the next three days could each be 10.
The sum is 6 + 30 + 12 = 48. The average is 48 / 7 = just under 7, but bigger than 6. The numbers cannot be made any smaller—you have to have a minimum of 2 a day. Once you hit the median of 10 in the middle slot, you have to have something greater than or equal to the median for the remaining slots to the right.
The smallest possible average is still bigger than 6, so this statement is sufficient to answer the question. The correct answer is (B).
Oh, and the OG question is DS #121 from OG13. If you think you’ve got the concept, test yourself on the OG problem.
Key Takeaway: Draw Out Scenarios
(1) Sometimes, these scenarios are so elaborate that people are paralyzed. Pretend your boss just asked you to figure this out. What would you do? You’d just start drawing out possibilities till you figured it out.
(2) On Yes/No DS questions, try to get a Yes answer and a No answer. As soon as you do that, you can label the statement Not Sufficient and move on.
(3) After a while, you might have to go back to your boss and say, “Sorry, I can’t figure this out.” (Translation: you might have to give up and guess.) There isn’t a fantastic way to guess on this one, though I probably wouldn’t guess (E). The statements don’t look obviously helpful at first glance… which means probably at least one of them is!
In honor of the final season of Breaking Bad, we decided to put together our ultimate Breaking Bad GMAT quiz. Those of you who fall in the overlapping section of the “Breaking Bad Fan” “GMAT student” Venn diagram should test your skills below… yo!
1. Data Sufficiency
Does x+4 = Walter White?
(1) x+4 is the danger
(2) x+4 is the one who knocks
A. Statement (1) ALONE is sufficient, but statement (2) ALONE is not sufficient
B. Statement (2) ALONE is sufficient, but statement (1) ALONE is not sufficient
C. BOTH statements TOGETHER are sufficient, but NEITHER statement ALONE is sufficient
D. EACH statement ALONE is sufficient
E. Statement (1) and (2) TOGETHER are NOT sufficient to answer the question asked, and additional data are needed
2. Discrete Quant
The front portion of Walter White’s Roof is a 7 ‘ by 15’ rectangle. If the diameter of a pizza is 22”, what is the approximate area of the shaded region of this diagram?
A. 13,600 inches sq.
B. 14,740 inches sq.
C. 15,120 inches sq.
D. 15,500 inches sq.
E. 16,640 inches sq.
3. Critical Reasoning
Today, Walter White will cook 100 pounds of methamphetamine.
This argument is flawed primarily because:
A. Cooking methamphetamine presents a moral dilemma for Walter White.
B. Walter White has to prioritize the needs of his wife and children and be a better father.
C. Walter has already paid for his cancer treatment and no longer needs to cook methamphetamine.
D. There is a fly in the laboratory.
E. He was told not to cook that day and is obeying his instructions.
4. Critical Reasoning
Hank’s collection of rocks includes over 400 different items. Hank’s rock collection is clearly the most impressive in New Mexico.
This argument is flawed primarily because:
A. Rock collections are not judged by the total number of rocks but by the rarity of each item included.
B. Rock collections are not impressive to anyone.
C. Hank’s rock collection is a metaphor and therefore cannot be judged against other rock collections.
D. Hank’s wife stole most of the rocks and it is therefore ineligible for any superlatives.
E. They aren’t rocks, they are minerals.
5. Discrete Quant
Walter Junior eats 3 eggs for breakfast every morning. Given that Walter Junior never misses breakfast, how many eggs does Walter Junior consume in March?
Answers are after the jump…