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GMAT Tip of the Week: Common Quant Mistakes That You Must Avoid

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GMAT Tip of the Week: 5 Common Quant Section Mistakes That You Must Avoid

BY Brian Galvin, VERITAS PREP


Much of your GMAT preparation will focus on “more” – learning more content, memorizing more rules, feeling more comfortable with the test format, and ultimately getting more questions right. But might impact your score more than “more” is your emphasis on “less” (or “fewer”). Feeling less anxiety, taking less time on tricky problems, having to guess less than in your previous attempts, and this ever-important concept:

Making fewer mistakes.

On an adaptive test like the GMAT, making silly mistakes on problems that you should get right can be devastating to your score. Not only do you get that question wrong, but now you’re being served easier questions subsequent to that, with an even more heightened necessity of avoiding silly mistakes there. So you should make a point to notice the mistakes you make on practice tests so that you’re careful not to make them again. Particularly under timed pressure in a high-stress environment we’re all susceptible to making mistakes. Here are 5 of the most common so that you can focus on making fewer of these:

1) Forgetting about “unique” numbers.

If someone asked you to pick a number 1-10, you might pick 5 or 6, or maybe you’d shoot high and pick 9 or low and pick 2. But you probably wouldn’t respond with 9.99 or 3 and 1/3. We tend to think in terms of integers unless told otherwise. Similarly, if someone asked “what number, squared, gives you 25″ you’d immediately think of 5, but it might take a second to think of -5. We tend to think in terms of positive numbers unless told otherwise.

On the GMAT, a major concept you’ll be tested on is your ability to consider all relevant options (an important skill in business). So before you lock in your answer, ask yourself whether you considered: positive numbers (which you naturally will), negative numbers, fractions/nonintegers, zero, the biggest number they’d let you use, and the smallest number they’d let you use.

2) Answering the wrong question.

An easy way for the GMAT testmaker to chalk up a few more incorrect answers on the problem is to include an extra valuable or an extra step. For example, if a problem asked:

Given that x + y = 8 and that x – y = 2, what is the value of y?

You might quickly use the elimination method for systems of equations, stacking the equations and adding them together:

x + y = 8

x – y = 2

2x = 10

x = 5

But before you pick “5” as your answer, reconsider the question – they made it convenient to solve for x, but then asked about y. And in doing so, they baited several test-takers into picking 5 when the answer is 3. Make sure you always ask yourself whether you’ve answered the right question!

Check more in PART II

3) Multiplying/dividing variables across inequalities.

By the time you take the test you should realize that if you multiply or divide both sides of an inequality by a NEGATIVE number, you have to flip the sign. -x > 5 would then become x < -5. But the testmakers also know that you’re often trained mentally to only employ that rule when you see the negative sign, –

To exploit that, they may get you with a Data Sufficiency question like:

Is a > 5b?

(1) a/b > 5

And many people will simply multiply both sides of statement 1’s equation by b and get to an ‘exact’ answer: a > 5b. But wait! Since you don’t know whether b is positive or negative, you cannot perform that operation because you don’t know whether you have to flip the sign. When you see variables and inequalities, make sure you know whether the variables are negative or positive!

4) Falling in love with the figure.

On geometry questions, you can only rely on the figure’s dimensions as fairly-reliable measurements if: One, it’s a Problem Solving question (you can never bring in anything not explicitly provided on a DS problem); and, two, if the figure does not say “not drawn to scale”. But if it’s a Data Sufficiency problem *or* if the figure says not drawn to scale, you have to consider various ways that the angles and shapes could be drawn. Often times people will see a “standard” triangle with all angles relatively similar in measure (around 60 degrees, give or take a few), and then base all of their assumptions on their scratchwork triangle of the same dimensions. But wait – if you’re not told that one of the angles could be, say, 175 degrees, you could be dealing with a triangle that’s very different from the one on the screen or the one on your scratchwork. Don’t get too beholden to the first figure you see or draw – consider all the options that aren’t prohibited by the problem.

5) Forgetting that a definitive “no” answer to a Data Sufficiency question means “sufficient.”

Say you saw the Data Sufficiency prompt:

Is x a prime number?

1) x = 10! + y, where y is an integer such that 1 < y < 10

Mathematically, you should see that since every possible value of y is a number that’s already contained within 10 * 9 * 8 * 7 * 6 * 5 * 4 * 3 * 2 * 1, whatever y is the new number x will continue to be divisible by. For example, if y = 7, then you’re taking 10!, a multiple of 7, and adding another 7 to it, so the new number will be a multiple of 7.

Therefore, x is not a prime number, so the answer is “no.” But here’s where your mind can play tricks on you. If you see that “NO” and in your mind associate that with “Statement 1 — NO”, you might eliminate statement 1 when really statement 1 *is* sufficient. You can guarantee that answer that x is not prime, so even though the answer to the question is “no” the statement itself is “positive” in that it’s sufficient.

So be careful here – if you get a definitive “NO” answer to a statement, don’t cross it out or eliminate it!

Remember, a crucial part of your GMAT study plan should be making fewer mistakes. While you’re right to seek out more information, more practice problems, and more skills, “fewer” is just as important on a test like this. Make fewer of the mistakes above, and your score will take you more places.
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Collection of Questions:
PS: 1. Tough and Tricky questions; 2. Hard questions; 3. Hard questions part 2; 4. Standard deviation; 5. Tough Problem Solving Questions With Solutions; 6. Probability and Combinations Questions With Solutions; 7 Tough and tricky exponents and roots questions; 8 12 Easy Pieces (or not?); 9 Bakers' Dozen; 10 Algebra set. ,11 Mixed Questions, 12 Fresh Meat

DS: 1. DS tough questions; 2. DS tough questions part 2; 3. DS tough questions part 3; 4. DS Standard deviation; 5. Inequalities; 6. 700+ GMAT Data Sufficiency Questions With Explanations; 7 Tough and tricky exponents and roots questions; 8 The Discreet Charm of the DS; 9 Devil's Dozen!!!; 10 Number Properties set., 11 New DS set.


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Re: GMAT Tip of the Week: Common Quant Mistakes That You Must Avoid [#permalink]

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The Most Common Wrong Answer to Any GMAT Problem

By Brian Galvin, VERITAS PREP


The GMAT is more than just a math or verbal test – it’s a reasoning test.  And so it’s important to think not merely about content, but also about the strategy games that the authors of these questions play with that content.  One mantra to keep in mind is “Think Like the Testmaker”, reminding yourself to pay just as much attention to why the wrong answer you chose was tempting (how did the author trick you) as to why the correct answer was right.

Arguably the single most common trap the authors set for you is evident in this question, which we invite you to answer before you read the rest of this post:

Uncle Bruce is baking chocolate chip cookies. He has 36 ounces of dough (with no chocolate) and 15 ounces of chocolate. How much chocolate is left over if he uses all the dough but only wants the cookies to consist of 20% chocolate?

(A) 3
(B) 6
(C) 7.2
(D) 7.8
(E) 9

Now, we don’t want to gloss over the math here but there’s plenty of opportunity to practice with word problems and ratios in other posts and resources, so let’s cut to the true takeaway here.  Most students will correctly arrive at the amount of chocolate used by employing a method similar to:

If the 36 ounces of dough are to be 80% of the total weight, then 36 = 4/5 * total.

That means that the total weight is 45 ounces, and so when we subtract out the 36 ounces of dough, there’s 9 ounces of chocolate in the cookies.

So…the answer is E. Right?

Wrong.  Go back and double-check the question – the question asks for how much chocolate is LEFT OVER, not how much is USED.  To be correct, you’d need to go back to the 15 original ounces of chocolate, subtract the 9 used, and correctly answer that 6 were left.

What’s the trap?  GMAT questions are frequently set up so that you can answer the wrong question.  If a question asks you to solve for y, it typically makes it easier to first solve for x…and then x is a trap answer.  If a question asks you to strengthen a conclusion, the best way to weaken it is likely to be an answer choice.  If a question asks for the maximum value, the minimum is going to be a trap.

The most common wrong answer to any problem on the GMAT is the right answer to the wrong question.

This question is discussed HERE.

So take precaution – to avoid this trap, make sure that you:

  • Circle the variable for which you’re solving, or write down the question at the top of your work.
  • Jot a question mark at the top of your noteboard on test day, and tap it with your pen before you submit your answer to double check “did I answer the right question?”
  • Keep track of your units in word problems (minutes vs. seconds, amount used vs. amount remaining) and double check the units of your answer against the question
  • Make note of every time you make that mistake in practice, and as a more general tip be sure not to write off silly mistakes as just “silly mistakes”.  If you made them in practice, you’re susceptible to them on the test, so make a note to watch out for them particularly if you’ve made the same mistake twice.


Few outcomes are more disappointing than doing all the work correctly but still getting the question wrong. The GMAT doesn’t do partial credit, so on a question like this falling for the trap is just as bad as not knowing how to get started.  Get credit for what you know how to do – make sure you pause before you submit your answer to make sure that it answers the proper question!
_________________

New to the Math Forum?
Please read this: Ultimate GMAT Quantitative Megathread | All You Need for Quant | PLEASE READ AND FOLLOW: 12 Rules for Posting!!!

Resources:
GMAT Math Book | Triangles | Polygons | Coordinate Geometry | Factorials | Circles | Number Theory | Remainders; 8. Overlapping Sets | PDF of Math Book; 10. Remainders | GMAT Prep Software Analysis | SEVEN SAMURAI OF 2012 (BEST DISCUSSIONS) | Tricky questions from previous years.

Collection of Questions:
PS: 1. Tough and Tricky questions; 2. Hard questions; 3. Hard questions part 2; 4. Standard deviation; 5. Tough Problem Solving Questions With Solutions; 6. Probability and Combinations Questions With Solutions; 7 Tough and tricky exponents and roots questions; 8 12 Easy Pieces (or not?); 9 Bakers' Dozen; 10 Algebra set. ,11 Mixed Questions, 12 Fresh Meat

DS: 1. DS tough questions; 2. DS tough questions part 2; 3. DS tough questions part 3; 4. DS Standard deviation; 5. Inequalities; 6. 700+ GMAT Data Sufficiency Questions With Explanations; 7 Tough and tricky exponents and roots questions; 8 The Discreet Charm of the DS; 9 Devil's Dozen!!!; 10 Number Properties set., 11 New DS set.


What are GMAT Club Tests?
Extra-hard Quant Tests with Brilliant Analytics

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Re: GMAT Tip of the Week: Common Quant Mistakes That You Must Avoid [#permalink]

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Re: GMAT Tip of the Week: Common Quant Mistakes That You Must Avoid [#permalink]

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New post 22 Jul 2015, 08:59
Awesome Post!!! Are there any more Threads/Docs on topics similar to the above ?

Cheers

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Re: GMAT Tip of the Week: Common Quant Mistakes That You Must Avoid [#permalink]

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Re: GMAT Tip of the Week: Common Quant Mistakes That You Must Avoid [#permalink]

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New post 22 Jul 2015, 14:14
hey bunuel... i read this awesome post to recognize our faults... i have a big problem these days.

on my first GMAT i scored 580 (Q49, V21), on second GMAT i scored 690(Q48, V35).
As im planning for the third attempt, i solved veritas free mock test and scored Q46.
Being disappointed, I solved the free GMATCLUB quants test and scored Q45.

I cant understand why my score is falling. Ive always been good in maths and am pretty much confident about my concepts.
I agree that DS gives me a hard time but im comfortable with even the odd topics like functions and probability.

Can u please suggest me as to why my performance is falling?

Thanks for reading.

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Re: GMAT Tip of the Week: Common Quant Mistakes That You Must Avoid [#permalink]

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Re: GMAT Tip of the Week: Common Quant Mistakes That You Must Avoid [#permalink]

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New post 29 Oct 2017, 21:01
Hello from the GMAT Club BumpBot!

Thanks to another GMAT Club member, I have just discovered this valuable topic, yet it had no discussion for over a year. I am now bumping it up - doing my job. I think you may find it valuable (esp those replies with Kudos).

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Re: GMAT Tip of the Week: Common Quant Mistakes That You Must Avoid   [#permalink] 29 Oct 2017, 21:01
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