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Avoiding Common Traps in GMAT Data Sufficiency Questions
1. Avoid Assumptions
2. Watch for Contradictions
3. Misapplication of the n Variable/n Equations Rule
4. Ignoring the Constraints on Variables
5. "Twin Trouble": Be Wary of Identical Equations
6. Beware of 0
7. Recognizing 'No' as an Indication of Sufficiency
8. The C-Trap Reexamined
9. "Spy Girl": Maintain Skepticism
10. Overcalculation in Data Sufficiency
11. Bias Against Certain Answer Choices
Conclusion
Mastering Data Sufficiency in the GMAT demands a nuanced approach, including an understanding of mathematical principles and an awareness of potential traps. By adopting these strategies, you can navigate this section more effectively, enhancing your overall performance on the GMAT.
1. Avoid Assumptions
- Trap: Making assumptions about a variables that aren't explicitly provided.
- Avoidance: Do not assume any conditions about values unless they are clearly stated. For example, don't presume a number is an integer or positive unless specified.
2. Watch for Contradictions
- Trap: Ignoring contradictions between statements.
- Avoidance: Remember, on the GMAT, two data sufficiency statements always provide true information and these statements never contradict each other or the stem. Ensure both statements come to the same logical conclusion. If one statement says x = 2 and another says x = 10, recheck your calculations. If you are getting a definite YES from one statement and a definite NO from another, reevaluate your logic!
3. Misapplication of the n Variable/n Equations Rule
- Trap: The common misconception is thinking that merely having n equations for n variables is enough to solve the system.
- Truth: The key is that these equations must be linear and distinct. This means each equation should provide unique information and not be derivable from the others through basic arithmetic operations.
- Avoidance: Carefully assess whether the given equations are both linear and distinct. Understanding this nuance is crucial in determining whether you can actually solve for all the variables in Data Sufficiency questions.
4. Ignoring the Constraints on Variables
- Trap: Often in solving GMAT Data Sufficiency problems, there's a tendency to overlook constraints on the variables. These constraints might include specific conditions like variables being integers, positive, or within a certain range. Forgetting these constraints can lead to incorrect assumptions and conclusions about the solvability of a problem.
- Avoidance: Always pay close attention to any given constraints on the variables. These limitations often play a crucial role in determining the sufficiency of data. Remember that variables might have inherent restrictions based on the context of the problem or explicitly stated in the question stem. This careful consideration can be the key to correctly solving many Data Sufficiency questions.
5. "Twin Trouble": Be Wary of Identical Equations
- Trap: Misinterpreting equations that are essentially the same.
- Avoidance: Understand that equations like x = 2 and 2x = 4 are not providing distinct information. In such cases, the answer is either each statement alone is sufficient (D), or both are insufficient (E).
6. Beware of 0
- Trap: Overlooking the significance of zero in calculations.
- Avoidance: Remember that zero is an integer and can be a critical number in your calculations. Also, consider the impact of dividing a smaller number by a larger one.
7. Recognizing 'No' as an Indication of Sufficiency
- Trap: Misconception arises when a 'no' answer to a Data Sufficiency question is perceived as inadequate or insufficient. This can lead to wrongly dismissing a valid conclusion.
- Avoidance: It's essential to understand that a definitive 'no' is as valid as a 'yes' in establishing sufficiency. The critical factor in GMAT Data Sufficiency is not whether the answer is positive or negative, but whether the data provided conclusively supports that answer. A consistent 'no' can effectively fulfill the question's requirement, confirming the sufficiency of the data.
8. The C-Trap Reexamined
- Trap: Quickly selecting 'C' when it seems both statements together are sufficient.
- Avoidance: Be cautious with option 'C'. If it seems clearly sufficient and options A & B don't, reevaluate. Attempt to validate 'C' before choosing it.
9. "Spy Girl": Maintain Skepticism
- Trap: Falling for answers that seem too easy or too good to be true.
- Avoidance: Approach each question with a degree of skepticism. If an answer seems straightforward, especially on higher-level questions, reconsider it.
10. Overcalculation in Data Sufficiency
- Trap: Engaging in unnecessary detailed calculations in Data Sufficiency questions, attempting to find specific numerical answers.
- Avoidance: Halt further calculations once the sufficiency of the given information is clear. The aim is to evaluate data sufficiency, not to solve for exact values. This approach is more time-efficient and aligns with the objective of these questions.
11. Bias Against Certain Answer Choices
- Trap: Hesitation in selecting "C" (both statements together are sufficient) or "E" (information is insufficient) in Data Sufficiency questions due to a bias against these options.
- Avoidance: Treat all answer choices as equally viable. Don't let a preconceived bias against options "C" or "E" sway your judgment. Each choice has an equal probability of being correct, and your decision should be based solely on the information provided in the question.
Conclusion
Mastering Data Sufficiency in the GMAT demands a nuanced approach, including an understanding of mathematical principles and an awareness of potential traps. By adopting these strategies, you can navigate this section more effectively, enhancing your overall performance on the GMAT.
Hi friends,
I open this thread because, I think, recognizing a pattern is very helpful in Math, expecially in DS with a very tricky problems. When we are in the battle, time is a gold.
In one long post of Traps Collection of the forum, I found that there was no general discription to differentiate the trap called SPY GIRL. So, if you dont mind, let paste here any problem has E choice as correct one. From that, may we have a rule? what do you think?
I am first.
I. SPY GIRL
1.
What fraction of this year's graduating students at a certain college are males?
a. Of this year's graduating students, 33 percent of the males and 20 percent of the females transferred from another college.
b. Of this year's graduating students, 25 percent transferred from another college.
AC=E
II THE TWIN
1.
Martha bought an armchair and a coffee table at an auctio and sold both items at her store. Her gross profit from the purchase and sale of the armchair was what percent greater than her gross profit from the purchase and sale of the coffee table?
a. Marth paid 10 percent for the armchair than for the coffee table
b. Marth sold the armchair for 20 percent more than she sold the coffee table
AC=E
2. Warehouse W's revenue from the sale of sofas was what percent greater this year than it was last year?
a. Warehouse W sold 10 percent more sofas this year than it did last year
b. Warehouse W's selling price per sofa was 30$ greater this year than it was last year.
AC=E
C-traps
1. Are at least 10 percent of the people in Country X who are 65 years older employed?
a. In Country X, 11.3 percent of the population is 65 years old or older
b. In Country X, of the population 65 years old or older, 20% of the men and 10% of the women are employed
AC =B
LET JOIN!
I open this thread because, I think, recognizing a pattern is very helpful in Math, expecially in DS with a very tricky problems. When we are in the battle, time is a gold.
In one long post of Traps Collection of the forum, I found that there was no general discription to differentiate the trap called SPY GIRL. So, if you dont mind, let paste here any problem has E choice as correct one. From that, may we have a rule? what do you think?
I am first.
I. SPY GIRL
1.
What fraction of this year's graduating students at a certain college are males?
a. Of this year's graduating students, 33 percent of the males and 20 percent of the females transferred from another college.
b. Of this year's graduating students, 25 percent transferred from another college.
AC=E
II THE TWIN
1.
Martha bought an armchair and a coffee table at an auctio and sold both items at her store. Her gross profit from the purchase and sale of the armchair was what percent greater than her gross profit from the purchase and sale of the coffee table?
a. Marth paid 10 percent for the armchair than for the coffee table
b. Marth sold the armchair for 20 percent more than she sold the coffee table
AC=E
2. Warehouse W's revenue from the sale of sofas was what percent greater this year than it was last year?
a. Warehouse W sold 10 percent more sofas this year than it did last year
b. Warehouse W's selling price per sofa was 30$ greater this year than it was last year.
AC=E
C-traps
1. Are at least 10 percent of the people in Country X who are 65 years older employed?
a. In Country X, 11.3 percent of the population is 65 years old or older
b. In Country X, of the population 65 years old or older, 20% of the men and 10% of the women are employed
AC =B
LET JOIN!
I've been working with the Advanced Quant guide by ManhattanGMAT for some time now and really benefited a lot from their excellent strategy guide on DS questions. I decided to summarize the basic steps of their DS section with an emphasis on what I think to be the most important points and combined this with my own experiences. I thought this might be beneficial to other users as well. I do not intend to violate any copyrights, I will merely focus on the basic principles without using any of the examples. The Advanced Quant Strategy guide is excellent and I think you take the most from it by really going through all the "Try-It" examples and all the questions in the back of the book. The book has a lot more to offer than the DS section and the other sections will definitely make you a smarter and better test taker, so buy the book if you find this helpful.
Those are in my opinion the most important stategic points to remember for all DS questions. There a couple of more points in the guide, but I didn't think they were that helpful.
DS Strategy:
- Make sure to write out all the important information in the question stem, whether it be certain (integer, pos/neg etc.) constraints, equations or the actual question. I noticed that this definitely helps you to prevent a lot of silly mistakes, such as forgetting about a constraint that is crucial to answering the question, but it also takes a lot of time. In my opinion it's best to try to pick up the constraints and then keep them in the back of your mind. When you analyze each statement, quickly get back to the question stem and look for the constraints, you save a couple of seconds this way and avoid silly mistakes due to forgetting about important information as good as possible.
- Always rephrase any equations, inequalities, fractions etc. Most of the simplifications that the GMAT uses are of the same pattern. For example try to write expressions with exponents in such a way that all of the bases are the same, or whenever you see a sum with root contained in it, try to multiply by its conjugate. The more problems you do the more you become accustomed to the GMAT style of simplifying expressions.
- Separate the statements on your paper cleary. I think this is really important, because statement carryover is a lot more likely, if you don't do it. Also you should always actively think about the danger of statement carryover, that way you are less likely to do it.
- Be a contrarian. You always have to look at all the possible scenarios when analyzing the statements, one trick that sometimes help you to detect that a statement is insufficient is to look at the other statement and deliberately try to violate that statement. I found this tip to be really helpful, because a lot of the times the opposite of the other statement is exactly the scenario that proves insufficiency.
- Assume nothing. On most questions where there aren't any constraints given, people sometimes still assume that there is e.g an integer constraint and thus only test integer cases. By really paying attention to what is given and whats NOT given, you can find the correct answer on a lot of questions.
- Compute the completion. When people see quadratic expressions, they sometimes assume that there are two solutions and that this proves insufficiency, since you can't find a single solution. This is a big mistake, on some occasions the value you are looking for is a product of two variables and plugging the two solutions in the product you actually get the same result. I found myself commiting this mistake quite often. You should always finish your calculation, unless you're 100% sure that you're correct. You don't want to spend a lot of time on a question, only to find yourself getting the question wrong, because you skipped the last calculation step, which would have cost you maybe 10 seconds.
- Beware of inequalities. Inequalities are really important in DS questions. You should especially look out for inequalities that combine with integer constraints, this will often give you the solution. If you forget about the constraint, the inequality in the statement might be of little use. Also look out for variables in the denominator in an inequality, never multiply the inequality by a variable, unless you know the sign of the variable. The GMAT uses this rule a lot in inequality DS questions.
- Use a scenario chart. In some problems there are only a handful of different options, this is especially true in positive/negative and even/odd type of questions. Create a table with each variable and each statement in it's own column and different scenarios in each row. Then pick numbers that satsify the given statements and see if there's only one solution to the question or multiple solutions. As soon as you find two possible solutions the statement is insufficient and you can go on to the next statement. If both statements are insufficient, use only those values for the variables that satsify both statements.
- Test numbers systematically. I think this is the most important skill in DS questions. Obviously you need to know all the basics to perform well on this test and it's always good to be able to prove sufficiency or insufficiency elagantly in an algebraic way. But sometimes a statement is just too difficult to handle algebraically or it takes too much time to solve algebraically. No matter how difficult a statement is, you can always pick numbers to prove sufficiency or insufficiency. But don't just test numbers randomly, you should have a systematic approach to it. MGMAT suggests to always have a standard set of numbers, which can be adjusted to the constraints given in the question and statement. The numbers are -2,-3/2,-1,-1/2,0,1/2,1,3/2 and 2. On some occasions it's also crucial to pick boundary values, e.g if a question ask whether x is less than or equal to 1 and a statement gives you x² less than or equal to 1.3, you can e.g pick x=1.1 to prove insufficiency.
Those are in my opinion the most important stategic points to remember for all DS questions. There a couple of more points in the guide, but I didn't think they were that helpful.
18 Mar 2009, 17:37
Kudos
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I was digging through my files today and found a doc I wrote a long time ago about Data Sufficiency (DS) Traps - I hope this helps someone. Perhaps someone can add more examples to make this thread more complete.
Data Sufficiency Traps
1. At the first glance there is not enough information to solve the problem, but when actually attempted, the problems appears solvable.
2. Both statements 1 and 2 are identical but masked.
3. Do not assume anything on data sufficiency.
4. What GMAT often does on harder DS questions, it gives the first piece of info as insufficient, and then, naturally, you move on to the second and then when you see that it is sufficient, you conclude that both are enough – C. Be careful to evaluate both statements one by one. If it is hard for you, try starting from the second statement instead of the first.
5. When you solve a medium/hard DS question, play a game with GMAT, find the answer to the puzzle. If you looked through the both pieces of info and it seems both are sufficient, try proving that one is irrelevant or if both pieces are needed, try to prove that only one will be enough; try challenging the question author; give him a hard time. It will often pay off.
6. Watch out for Yes/No data sufficiency questions.
7. Make an analysis of your mistakes and see what DS questions cause the most problems.
8. Make sure you don’t confuse D and C answer choices
Data Sufficiency Traps
1. At the first glance there is not enough information to solve the problem, but when actually attempted, the problems appears solvable.
- What is the volume of a box with dimensions a, b, and c?
i. a = \(\frac{18}{bc}\)
ii. b = 2, c = 4
2. Both statements 1 and 2 are identical but masked.
- What is the value of x?
i. x + 2y = 6
ii. 4y + 2x = 12
3. Do not assume anything on data sufficiency.
- On January 1st Thomas deposited $2,000 into an interest bearing checking account. If he made no withdrawals, what was the total amount Thomas had in the checking account on December 31st of the same year?
i. Thomas deposited an additional $4,000 throughout the year
ii. The checking account earned 7 percent simple interest
You cannot assume that the deposits were uniform or in equal installments
How many kiloliters of water are in a reservoir?
i. If the reservoir were filled to capacity, there would be 430 more kiloliters in the reservoir.
ii. The reservoir is normally 65 percent full.
We don't know if these are "normal" conditions. Insufficient.
4. What GMAT often does on harder DS questions, it gives the first piece of info as insufficient, and then, naturally, you move on to the second and then when you see that it is sufficient, you conclude that both are enough – C. Be careful to evaluate both statements one by one. If it is hard for you, try starting from the second statement instead of the first.
5. When you solve a medium/hard DS question, play a game with GMAT, find the answer to the puzzle. If you looked through the both pieces of info and it seems both are sufficient, try proving that one is irrelevant or if both pieces are needed, try to prove that only one will be enough; try challenging the question author; give him a hard time. It will often pay off.
6. Watch out for Yes/No data sufficiency questions.
7. Make an analysis of your mistakes and see what DS questions cause the most problems.
8. Make sure you don’t confuse D and C answer choices
