Lesson: Double Matrix Method

Comment on Double Matrix Method

Thank you for this straightforward method, it's really helpful.
greenlight-admin's picture

Thanks for taking the time to say that!

In a survey,it was found that 10% of the students who are susceptible to tuberculosis are less than 20 years of age and 40% of the students who are not susceptible to tuberculosis are more than or equal to 20 years of age.If 9% of the total students are more than or equal to 20 years of age and are susceptible to tuberculosis,then what percentage of the students,that are 20 years old or more,are not susceptible to tuberculosis?
greenlight-admin's picture

Hi Hari,

Sorry, but it's next to impossible for me to use the limited functionality of this comment board to demonstrate the Double Matrix method in solving your question.

You might want to post this question on GRE Prep Club.

Hi Brent!, Thanks for this awesome approach.
i am confused in the below question

http://greprepclub.com/forum/qotd-11-of-the-students-in-a-school-20-percent-are-in-the-2660.html
and
http://greprepclub.com/forum/if-75-percent-of-the-employees-of-a-certain-company-take-a-w-2701.html

in first question, it ask for what percent of the students who are in the science club are not in the band?

i answered 15per, but in your explanation you have given 75 per. 15per is not even choices.

in second question, it ask for what percent of the employees take a summer vacation but not a winter vacation?

though i was thinking 5 per, i answered as (5/45) * 100, as i had gone through in last question's answer explanation.

what am i doing wrong?, what is the style difference between those question?
greenlight-admin's picture

For the first question (http://greprepclub.com/forum/qotd-11-of-the-students-in-a-school-20-perc...), you are answering a different question. You are answering a question that asks "What percent of the ENTIRE school population is in the science club but not in the band?"

The actual question asks "What percent of the SCIENCE CLUB STUDENTS are not in the band?"

There are 20 students in the science club. Of those 20 students, 15 are NOT in the band.

15/20 = 3/4 = 75%

In the second question (http://greprepclub.com/forum/if-75-percent-of-the-employees-of-a-certain...), you answered a different question as well.

You answered the question "What percent of THE SUMMER VACATIONERS do not take a winter vacation?"

The actual question asks "What percent of ALL EMPLOYEES take a summer vacation but do not take a winter vacation?"

I can see how the first question could have thrown you off. It's a very common mistake. The difference is wording is quite subtle and easy to miss.

Nice and clear method! Thanks.

Hi Brent, thanks for share with us this great method. This method only works for problems that have Overlapping sets. For example can be used for the following problems ?http://greprepclub.com/forum/set-a-consists-of-40-integers-1893.html and
http://greprepclub.com/forum/in-a-graduating-class-of-236-students-142-took-algebra-and-1815.html
Thanks so much.

Hi Brent!

I have come across the below question in ETS quant guide

Set A has 50 members and set B has 53 members. At least 2 of the members in set A are not in set B. Which of the following could be the number of members in set B that are not in set A?

indicate all such numbers

a)3 b)5 c)13 d)25 e)50 f)53

my approach:
min number of members from A can be in B are : 2
max number of members from A can be in B are : 48

with min value 5 number of members of B who are not in A.
with max value 51 number of members of B who are not in A.

so answers would be B C D E

but in official guide F is also in answer, can you please explain.
greenlight-admin's picture

Hi yogasuhas,

I have responded to this question here: http://greprepclub.com/forum/set-a-has-50-members-and-set-b-has-53-membe...

Hello Brent,
In the following question, I confuse in understanding the statement ' the number of people who graduated from college in 1997 who did not both receive a degree in the applied sciences and graduate from a school in one of six northeastern states?'

I thought that we need to find the value of last square (Not AS & Not NES). However, we need to find the value of the three blue shaded squares.

Could you please once explain that statement for a better understanding ?

http://greprepclub.com/forum/in-1997-n-people-graduated-from-college-if-1-3-of-them-rec-2700.html

Thanks :-)
greenlight-admin's picture

Question link: http://greprepclub.com/forum/in-1997-n-people-graduated-from-college-if-...

Let me answer your question with an analogous story.

Let's say that, when you left for work, there WERE two pieces of pie in the fridge. When you came home, you found that both pieces are missing.

If ask your friend "Did you eat BOTH pieces of pie?" and he/she says, "No, I did NOT eat BOTH pieces of pie," what can you conclude from this answer?

Does it mean your friend did not eat ANY pie?

No. All you can conclude is that your friend did not eat BOTH pieces. However, if your friend ate ONE piece of pie, then he/she would be telling the truth when replying "No, I did NOT eat both pieces of pie."

So, when your friend says "I did NOT eat both pieces of pie," this means EITHER your friend ate zero pieces of pie OR your friend at one piece of pie.

The question asks us to determine "the number of people who did NOT BOTH receive a degree in applied sciences AND graduate from a school in one of six northeastern states"

This includes:
- the students who received a degree in applied sciences but did NOT attend a school in one of six northeastern states"
- the students who did NOT receive a degree in applied sciences but DID attend a school in one of six northeastern states"
- the students who did NOT receive a degree in applied sciences and did NOT attend a school in one of six northeastern states"

Does that help?

pls solve the question in 3rd edition official gre guide 161(5)...i know its double matrix method
greenlight-admin's picture

hey brent Im having trouble assigning names to the box hence the entire concept gets lost on me. help!
greenlight-admin's picture

To apply the Double Matrix Method, we must identify that a certain population is such that each member has two characteristics associated with it.

So, for example, we may have a population of baseball players, and the two characteristics are:
- left-handed or right-handed
- speaks Spanish or does not speak Spanish

It may take time to identify (and label) the two characteristics.
I suggest that you carefully go through each of the linked questions above and try to identify the two characteristics.
Then review the responses of others.
This is the only way to master this technique.

Cheers,
Brent

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