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High-Level Math Help

Problem Solving — algebra and arithmetic (GMAT Focus Edition)
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High-Level Math Help

by SeanCohen » Tue Jul 21, 2015 7:31 am
I am preparing to take the GMAT test. I took a practice test today and scored Q:46, V:51 for 760 total.

Other than reading the Princeton Review 2013 book, which was pretty basic, I have done no real math study. I was quite good at math in high school. I scored 790 on the math portion of the SAT. That was years ago.

Generally speaking, my problem is speed. I'm too fast. I did 20 data sufficiency problems in 11:15, but I missed three. The questions just seem so easy -- and they are, but I'm right only 85 percent of the time. I've started double-reading the question to make sure I am solving for the right thing. That's cut my error rate down quite a bit.

I need Q:48+ no ifs, ands, or buts about it.

I have trouble with combinations and permutations. I'm not even sure what the difference is. I work the problems out with my fingers. For example, if the problem asks me how many groups of 3 can be formed with 5 people, I write down:

A B C D E

and using three fingers I start counting:

ABC, ABD, ABE, BCD, BCE, BDE, CDE. Seven, that's what I figure.

I remember reading somewhere that you are supposed to use 5! - 3! or something like that, but I never come up with the right answer that way.

Suggestions?
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by [email protected] » Tue Jul 21, 2015 7:45 am
Hi SeanCohen,

You bring up a number of different issues in this post, so I'm going to address them in the order that you brought them up:

1) First off, you're scoring at a fantastically high level on your CATs. When you take your CATs, how realistically do you make that 'process?' Do you take the ENTIRE CAT (including the Essay and IR sections)? Do you take them at home or an external location? Do you take the CATs at the same time of day as your Official GMAT? Etc.

2) When you scored the Q46, how many of those Quant questions did you get wrong because of a silly/little mistake? How many did you get wrong because you just did not know how to do the 'math' involved? Did you finish the Quant section early (and if so, then how many minutes early?)?

3) WHY do you think you need a Q48+?

4) There's nothing wrong with using 'brute force' math when dealing with certain Combination, Permutation or Probability questions, BUT you have to be thorough with your thinking.

In your example, there are actually 10 groups of three (not 7). They are:

ABC
ABD
ABE
ACD
ACE
ADE

BCD
BCE
BDE

CDE

In this situation, you can also use the Combination Formula:

N!/[K!(N-K)!] where N is the total number of items and K is the number in the 'subgroup'

5!/(3!2!) = 10 different combinations of 3 people

This formula, and the other salient points involved in these types of Quant questions, are things that you can learn IF you do the appropriate Quant prep.

When are you planning to take the GMAT?

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by SeanCohen » Tue Jul 21, 2015 8:47 am
Thanks for your fast response.

First of all, I don't know what a CAT is. Are you referring to computer-adaptive tests? As for how realistically (sic) I make the process, the test process is as realistic as I can make it. I am using older, paper-based tests from 1996. The tests start with two essays, and I write both. Then I do the three math sections together in 75 minutes. There are 52 questions total. I finished this section in about 48:25. I missed 3 data sufficiency, and 5 quant questions for a raw score of 42, scaled score of 46. This is too low. Additionally, I have been told that the current test has more demanding math questions on it. I need to score a 48 on the real test because I have to score in the top 20 percent to get a raise at my job. I'm a GMAT teacher at a no-name institute outside of the USA. I also teach ESL and TOEFL.

Here's a sample question I missed:

If it is 6:27 in the evening on a certain day, what time in the morning was it exactly 2,880,717 minutes earlier? (Assume standard time in one location.)

A) 6:22
B) 6:24
C) 6:27
D) 6:30
E) 6:32

Procedure followed:

First I multiplied 60 x 24 same as 120 x 12 = 1440, so the 2880 is irrelevant -- crossed off. 717 divided by 60 is about 11. 717 - 660 = 57, which is three minutes less than an hour. So it must be 6:24 as that's three minutes less than an hour.

However, the problem stated 2,880,717 minutes EARLIER and I calculated 2,880,717 minutes LATER. Oops.

I'd say about 80 percent boneheaded mistakes, and 20 percent I don't know how to do the problem. I am weakest at geometry, combinations, and permutations.

I have no specific time frame for taking the GMAT, but considering how well I'm scoring, I think I am getting close. Maybe in the next 30 days. The price of the test is a challenge for me. I think it's $250? That's almost three times my monthly rent. I need to nail this on the first try.
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by Brent@GMATPrepNow » Tue Jul 21, 2015 9:00 am
First of all, if you're interested, we have a free video on calculating combinations in your head: https://www.gmatprepnow.com/module/gmat-counting?id=789

When it comes to the 2nd question...
If it is 6:27 in the evening on a certain day, what time in the morning was it exactly 2,880,717 minutes earlier? (Assume standard time in one location.)

A) 6:22
B) 6:24
C) 6:27
D) 6:30
E) 6:32
We need only pay attention to the colored digits above.
We must subtract 2,880,717 minutes from 6:27 (aka 6 hours and 27 minutes)

When we do that, we get a time of XX:X0 (since 7 - 7 - 0)
Check the answer choices... only D works
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by [email protected] » Tue Jul 21, 2015 7:20 pm
Hi SeanCohen,

The GMAT is a Computer-Adaptive Test (CAT), so these pencil-and-paper test scores that you've earned are likely 'inflated' (since those tests are arguably easier than the current version of the GMAT). I suggest that you download the free software package from www.mba.com and take one of the CATs that comes with that package. With those score results, you'll have a much more realistic assessment of your abilities.

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by SeanCohen » Wed Jul 22, 2015 7:22 am
Brent@GMATPrepNow wrote:First of all, if you're interested, we have a free video on calculating combinations in your head: https://www.gmatprepnow.com/module/gmat-counting?id=789

When it comes to the 2nd question...
If it is 6:27 in the evening on a certain day, what time in the morning was it exactly 2,880,717 minutes earlier? (Assume standard time in one location.)

A) 6:22
B) 6:24
C) 6:27
D) 6:30
E) 6:32
We need only pay attention to the colored digits above.
We must subtract 2,880,717 minutes from 6:27 (aka 6 hours and 27 minutes)

When we do that, we get a time of XX:X0 (since 7 - 7 - 0)
Check the answer choices... only D works
I found the video quite informative. However, I have a question about the procedure. Below is a combinations question.

Five couples are part of a homeowner's association, and all of them are willing to serve on the board of directors, which consists of three members. HOA rules prohibit both members of a couple from serving on the board. How many possible combinations of board of directors are there?

If I understand your video, the answer should be:

10 x 8 x 6
----------
3 x 2 x 1

or 80.

Is that the right answer?
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by SeanCohen » Wed Jul 22, 2015 2:53 pm
[email protected] wrote:Hi SeanCohen,

The GMAT is a Computer-Adaptive Test (CAT), so these pencil-and-paper test scores that you've earned are likely 'inflated' (since those tests are arguably easier than the current version of the GMAT). I suggest that you download the free software package from www.mba.com and take one of the CATs that comes with that package. With those score results, you'll have a much more realistic assessment of your abilities.

GMAT assassins aren't born, they're made,
Rich
I downloaded the program in question and rushed through it. I was unprepared for IR, as I had never done that type of question before. I often couldn't even understand what the question was asking.

Some of the math questions were baffling for me. For example, there was a question about temp employees. Four of the employees would become permanent. 5 out of 12 were women, 7 out of 12 men. How many combinations were there of 3 women and 1 man selected to become permanent? I had no clue how to do that problem. I guessed and moved on.

I finished the quant section with 35 minutes to spare and the verbal with 30 minutes to spare. My score was:

IR - 4 - 37 percent
Q - 48 - 78 percent
V - 44 - 97 percent
Final - 740 - 97 percent

I'm certain I would have scored higher on the verbal had I bothered to read the reading passages.
The math score is unacceptable. I need to move it up.
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by SeanCohen » Wed Jul 22, 2015 4:54 pm
Nevermind. I figured out how to do that temp worker problem.
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by Ian Stewart » Fri Aug 07, 2015 9:18 pm
To clear one thing up from a post above - the computer version of the GMAT was calibrated against the paper tests, so the scores are perfectly comparable. GMAT questions are harder now than before (since ACT started developing the test), but you need to get fewer of the questions right to get the same score. But a Q48 today is supposed to represent the same level of ability as a Q48 fifteen years ago.

It sounds like you need to do a systematic review of the GMAT curriculum - if you're scoring ~Q48 now, you clearly know most of the material, but it also sounds as though there are gaps in your knowledge, at least in counting and probability, and possibly in other subjects. So I think what would help you most is finding a high-quality set of books that covers all of GMAT math, specifically written for high-level test takers (lower level books will waste your time), and skim through them, reading in detail those parts that aren't familiar to you now.

If you can fill in the gaps in your foundation, and continue to work to eliminate careless errors, it sounds like you have the potential to do extremely well (not many people are capable of getting a Q48 when only spending 45 mins on the Quant section!).
For online GMAT math tutoring, or to buy my higher-level Quant books and problem sets, contact me at ianstewartgmat at gmail.com

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by SeanCohen » Mon Aug 24, 2015 5:44 am
Dear Ian,

Thank you very much for writing to me. I regret not checking the forum till now.

I have taken two tests recently. The first one, a paper test, netted me a 790. I scored 50/52 on math. Both of my errors were in data sufficiency. Yesterday I took another one, a computer test, and I scored as follows:

IR: 7 = 81 percent
Q: 48 = 78 percent
V: 47 = 99 percent

T: 760 = 99 percent

I have been studying the quant section exclusively. I got ahold of the official guide 13, and I have done at least 80 percent of the calculation questions. I am somewhat dismayed to find that my score seems stuck at 48 computer test. I believe that my problem may be data sufficiency. I will attempt to study that in hopes of improving.

I'll send you a message privately about the books.
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by MartyMurray » Thu Aug 27, 2015 6:40 pm
Maybe you can get a clearer picture of how to do the combinations and permutations problems by reading this.

https://infinitemindprep.com/permutation ... difficult/

One thing you could do to get better at data sufficiency is to slow down in practice and develop a better sense of what it takes to get right answers.

Meanwhile, data sufficiency is all about catching things and verbal is all about seeing things, both big picture things and details.

Rushing through the test makes it much more difficult to see what you need to see.

You can probably solve most of if not all of your problems and hit your score target by just slowing down.

Do you drink coffee? If so maybe try drinking no coffee for 24 hours and see how you do on the test after that.
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