Double Matrix method

Expert replies

Mo2men: Legendary Member; Posts: 712; Joined: Fri Sep 25, 2015 4:39 amFollowed by: 5 members; Thanked: 14 times

by Mo2men » Thu Feb 02, 2017 3:45 am

Jack took an exam of 45 questions, how many questions did he answer correctly?

(1) The number he answered incorrectly is twice the number of questions he did not attempt at all.
(2) The total number of questions he answered incorrectly or did not attempt is 3/9 of all questions.

How I can use double matrix method? How statement 2 is interpreted when using 'or'?

I feel it is illogical to say un-attempted question is either correct or incorrect.

Thanks

Like this post

Source: Beat The GMAT — Data Sufficiency | Thu Feb 02, 2017 3:45 am

GMATGuruNY: GMAT Instructor; Posts: 15539; Joined: Tue May 25, 2010 12:04 pmFollowed by: 1906 members; Location: New York, NY; Thanked: 13060 times; GMAT Score:790

by GMATGuruNY » Thu Feb 02, 2017 5:11 am

Mo2men wrote:Jack took an exam of 45 questions, how many questions did he answer correctly?

(1) The number he answered incorrectly is twice the number of questions he did not attempt at all.
(2) The total number of questions he answered incorrectly or did not attempt is 3/9 of all questions.

For each question, there are 3 options:
Correct, Incorrect, Not Attempted.

Let C = correct, I = incorrect, N = not attempted.

Since the total number of questions = 45, we get:
C + I + N = 45.

Statement 1:
Case 1: N=1, I=2, C = 45-1-2 = 42.
Case 2: N=2, I=4, C = 45-2-4 = 39.
Since C can be different values, INSUFFICIENT.

Statement 2:
Since I + N constitute 3/9 of the 45 questions, C must constitute 6/9 of the 45 questions:
C = (6/9)45 = 30.
SUFFICIENT.

The correct answer is B.

Last edited by GMATGuruNY on Thu Feb 02, 2017 5:30 am, edited 1 time in total.

Private tutor exclusively for the GMAT and GRE, with over 20 years of experience.
Followed here and elsewhere by over 1900 test-takers.
I have worked with students based in the US, Australia, Taiwan, China, Tajikistan, Kuwait, Saudi Arabia -- a long list of countries.
My students have been admitted to HBS, CBS, Tuck, Yale, Stern, Fuqua -- a long list of top programs.

As a tutor, I don't simply teach you how I would approach problems.
I unlock the best way for YOU to solve problems.

For more information, please email me (Mitch Hunt) at [email protected].
Student Review #1
Student Review #2
Student Review #3

Like this post

GMATGuruNY: GMAT Instructor; Posts: 15539; Joined: Tue May 25, 2010 12:04 pmFollowed by: 1906 members; Location: New York, NY; Thanked: 13060 times; GMAT Score:790

by GMATGuruNY » Thu Feb 02, 2017 5:16 am

Mo2men wrote:How I can use double matrix method?

Because there is no overlap between correct and incorrect -- a question cannot be answered BOTH correctly and incorrectly -- I would not use a double-matrix.

How statement 2 is interpreted when using 'or'?

incorrect or not attempted = (total incorrect) + (total not attempted).

Like this post

Mo2men: Legendary Member; Posts: 712; Joined: Fri Sep 25, 2015 4:39 amFollowed by: 5 members; Thanked: 14 times

by Mo2men » Thu Feb 02, 2017 5:23 am

GMATGuruNY wrote:
For each question, there are 3 options:
Correct, Incorrect, Not Attempted.

Let C = correct, I = incorrect, N = not attempted.

Since the total number of questions = 45, we get:
C + I + N = 45.

Statement 1:
Case 1: N=1, I=2, C = 45-1-2 = 42.
Case 2: N=2, I=4, C = 45-2-4 = 39.
Since C can be different values, INSUFFICIENT.

Statement 2:
Since I + N constitute 3/9 of the 45 questions, C must constitute 2/9 of the 45 questions:
C = (2/9)45 = 10.
SUFFICIENT.

The correct answer is B.

Thanks for your response.

But i do not understand how you arrive to 'C must constitute 2/9 of the 45 questions'.

I think I+N= 3/9 *45 =15

So C= 30

Can you please clarify??

Like this post

GMATGuruNY: GMAT Instructor; Posts: 15539; Joined: Tue May 25, 2010 12:04 pmFollowed by: 1906 members; Location: New York, NY; Thanked: 13060 times; GMAT Score:790

by GMATGuruNY » Thu Feb 02, 2017 5:33 am

Mo2men wrote:I think I+N= 3/9 *45 =15

So C= 30

Can you please clarify??

Good catch.
My post above has been corrected to read as follows:
Since I + N constitute 3/9 of the 45 questions, C must constitute 6/9 of the 45 questions:
C = (6/9)45 = 30.

Like this post

Mo2men: Legendary Member; Posts: 712; Joined: Fri Sep 25, 2015 4:39 amFollowed by: 5 members; Thanked: 14 times

by Mo2men » Thu Feb 02, 2017 5:41 am

GMATGuruNY wrote:
Mo2men wrote: How statement 2 is interpreted when using 'or'?
incorrect or not attempted = (total incorrect) + (total not attempted).

If we use the word 'and' instead of 'or', does it make any difference?

Thanks for support

Like this post

GMATGuruNY: GMAT Instructor; Posts: 15539; Joined: Tue May 25, 2010 12:04 pmFollowed by: 1906 members; Location: New York, NY; Thanked: 13060 times; GMAT Score:790

by GMATGuruNY » Thu Feb 02, 2017 5:46 am

Mo2men wrote:
GMATGuruNY wrote:
Mo2men wrote: How statement 2 is interpreted when using 'or'?
incorrect or not attempted = (total incorrect) + (total not attempted).
If we use the word 'and' instead of 'or', does it make any difference?

Thanks for support

For a question to be answered incorrectly, it must be ATTEMPTED.
Thus, it is not possible for a question to be incorrect AND not attempted.

Like this post

Mo2men: Legendary Member; Posts: 712; Joined: Fri Sep 25, 2015 4:39 amFollowed by: 5 members; Thanked: 14 times

by Mo2men » Thu Feb 02, 2017 5:49 am

GMATGuruNY wrote:
For a question to be answered incorrectly, it must be ATTEMPTED.
Thus, it is not possible for a question to be incorrect AND not attempted.

Thanks a lot.

good catch. It is certainly illogical.

Like this post

Post new topic Post Reply

• Page 1 of 1

GMAT Course Reviews

560 reviews

1168 reviews

431 reviews

359 reviews

147 reviews

5/5

5 Star (560 Reviews)

"Target Test Prep is the closest to the official version of the GMAT exam, about 99% accuracy in terms of the quality and quantity of information. The course has excellently created singular sets of focused lessons and tests for every possible topic that one could come across in the official GMAT exam."

"The TTP course maximizes the efficiency of the time you spend studying. It will take time and effort but I could almost guarantee that if you complete the course exactly as it is laid out you will get an amazing score. They also have a very responsive team willing to help with any questions you might have."

"TTP has two things that I think no other test prep company offers: A teaching approach that reinforces understanding and an attitude that will give you the mental preparedness needed to succeed on the test. TTP gives you a deep understanding of the concept you need to know while teaching you how to think."

Write a Review