offers hundreds of practice questions and video explanations. Go there now.

Sign up or log in to Magoosh GMAT Prep.

GMAT Sets: Double Matrix Method

Learn this powerful method for unlocking devilishly complicated problems about sets. 

 

Practice questions

First, try these challenging 700-level practice questions.

1) At Veridux Corporation, there are 250 employees. Of these, 90 are female, and the rest are males.  There are a total of 40 managers, and the rest of the employees are associates.  If there are a total of 135 male associates, how many female managers are there?

A. 15
B. 20
C. 25
D. 30
E. 35

2) In the county of Veenapaniville, there are a total of 50 high schools, of three kinds: 25 public schools, 16 parochial schools, and 9 private independent schools.  These 50 schools are divided between three districts: A, B, and C. District A has 18 high schools total. District B has 17 high schools total, and only two of those are private independent schools.  If District C has an equal number of each of the three kinds of schools, how many private independent schools are there in District A?

A. 2
B. 3
C. 4
D. 5
E. 6

 

This kind of question

It would not be surprising if you found one or both of those questions totally bewildering.  Without the right method, this type of question can be a nightmare.

First of all, let’s be clear about what we mean by “this type of question.”  These are set questions in which each element, each member, is classified according to two different variables, and each one of those variables has two or more categories.  For example, in question #1, the two variables are:

a) gender (male or female)
b) employee type (manager or associate)

Every one of those 250 employees is categorized both in terms of gender and in terms of employee type.  That’s key: every member is categorized in two different ways.  This type of question is screaming for a method of solution called the Double Matrix Method.

 

The Double Matrix Method: example #1

I will demonstrate the Double Matrix Method first by solving question #1, then question #2.  In the Double Matrix Method, you make a sectioned box, a rectangle with subdivisions.  The horizontal subdivisions will represent the categories of one variable, and the vertical subdivisions will represent the categories of the other variable.  Here’s the set-up for question #1: I temporarily put letters in the boxes, to facilitate talking about those boxes.

A is number of male managers, B is the number of female managers, and A + B = C is the total number of managers.  Similarly, D is the number male associates, and A + D = G is the total number of males.  In any row, you can add across the first two boxes and the sum will be the box on the right.  In any column, you can add the top two boxes, and the sum will be the bottom box.  The final number, I, is the “grand total”, the total number of employees in the whole problem, which obvious has to equal the sum of males + females, as well as the sum of managers + associates.

Here’s that matrix again with the numbers from the problem filled in — just the numbers stated in black-and-white in the problem.

Well, first let’s take care of the “totals”.  The numbers in the “totals” row must add up.  If 90 are females, the other 250 – 90 = 160 must be males.  Similarly, the numbers in the “totals” column must add up.  If 40 are managers, then the other 250 – 40 = 210 must be associates.

Now, in the “associate” row, 135 + E = 210, which means E = 75 — the other 75 associates must be female.

Now, to find B, which is what the question is asking, we need only look at the sum in the “female” column: B + 75 = 90, which means B = 15.  There are fifteen female managers in this company.  Thus, the answer = (A).

Instead of this route, once we found all the “totals” numbers, we could have used addition in the “male” column to find A = 25, then use addition across the “managers” to find B = 15 —- in fact, if you have a second route for finding the number, you should perform that too, to verify that you get the same number both ways.

 

The Double Matrix Method: example #2

Question #2 is very difficult: it’s debatable whether a set question this difficult even would appear on the GMAT.  Nevertheless, the Double Matrix Method unlocks it easily.  First of all, our matrix now has to have three categories for each variable.

As before, in any row, the sum across the first three entries will equal the right-most entry in the “totals” column; and similarly, the sum down the top three entries in any column will equal the bottom entry in the “total” row; and, of course, all the totals, in either the “totals” row or the “totals” column, have to add up to the grand total in the lower right-hand box.  Let’s start filling this in, following the information given in the question.  “In the county of Veenapaniville, there are a total of 50 high schools, of three kinds: 25 public schools, 16 parochial schools, and 9 private independent schools.”

District A has 18 high schools total.”

District B has 17 high schools total, and only two of those are private independent schools.

District C has an equal number of each of the three kinds of schools.”  Hmm.  That’s not a simple number.  Let’s put that on hold for a moment.  So far, from the matrix it looks like we could figure out the missing number in the “totals” row.  That number must be 15, because 18 + 17 + 15 = 50.

Now, go back to that last fact: “District C has an equal number of each of the three kinds of schools.”  Now that makes sense.  District C has 15 schools, so it must have 5 of each kind:

The question is asking: “how many private independent schools are there in District A?”  Well, now that’s just a matter of adding across the “private independent” row.  We know 2 + 2 + 5 = 9, so the missing number must be 2.

There’s still a good deal of information we can’t figure out from the information given in the question, but we are able to answer the specific question asked here.  There are 2 private independent schools in District A, so the answer = (A).

Notice, incidentally, if the question were framed differently, as a DS question, the Double Matrix method would allow us to see what questions we could and couldn’t answer based on the information given.  In a DS question, one would have to write out one Double Matrix for Statement #1, and a whole new Double Matrix for Statement #2.  Trying to save time by cramming all the information from both statements into a single matrix is an excellent way to make the classic mistake of conflating the information given in the two statements, one of the most common mistakes on Data Sufficiency.

 

Practice question

Here’s another very challenging question that you should be able to unlock with the help of the Double Matrix Method.

3) http://gmat.magoosh.com/questions/11

 

Ready to get an awesome GMAT score? Start here.

Most Popular Resources

19 Responses to GMAT Sets: Double Matrix Method

  1. waqas May 11, 2016 at 3:07 am #

    you are awesome dude. Stay Blessed 🙂

  2. Charu January 29, 2016 at 8:07 am #

    Hi Mike,

    Thank you so so much for this lesson, particularly for teaching this trick!
    You have just opened my eyes and I am actually enjoying solving these problems.

    Best regards,

    Charu

    • Jitesh January 29, 2016 at 12:40 pm #

      Nice to see it’s helping you too Charu.

  3. Jitesh January 28, 2016 at 9:24 am #

    Dear Mike,
    Thank you very much for all your video lessons and posts. I really like the way you explain and make problems easy to solve. I wish you were my high school math teacher. I just hope I get my desired GRE score. Thanks!

    Regards,
    Jitesh (Premium Magoosh Student)

  4. Marco October 13, 2015 at 7:14 am #

    Hello! Can we use the Double Matrix Method for Venn Diagram questions?

  5. Bota October 8, 2015 at 11:59 pm #

    Hi Mike! Thanks for the post. Approximately how many problems that apply Venn diagrams and double matrix should a I expect during the GRE? I understand that it’s somewhat difficult to say, but a rough approximation would be very helpful – I love these problems and find them easier than other word problems.

    Also, I don’t apply the double matrix normally, opting for simply writing every category down with different branches (sounds confusing, I know). I’m just more comortable with this strategy. I can keep it this way as long as it’s faster and easier for me, right?

    Thanks!

    • Dani Lichliter
      Dani Lichliter October 9, 2015 at 9:57 am #

      Hi Bota!

      Great questions! Since you are a premium member, I went ahead and forwarded your questions on to our remote team of experts. Someone from that team will reach out to you via email.

      Happy studying! 🙂
      Dani

  6. Mohit Agrawal September 17, 2014 at 5:01 pm #

    Thanks Mike.

    That was a wonderful explanation. Not worried about these type of questions anymore. 🙂

    • Mike MᶜGarry
      Mike September 18, 2014 at 1:40 pm #

      Dear Mohit,
      You are quite welcome. 🙂 Best of luck to you!
      Mike 🙂

  7. paresh February 18, 2014 at 2:08 am #

    Hi Mike,

    wonderful definition of the Double Matrix Method

    • Mike MᶜGarry
      Mike February 18, 2014 at 10:03 am #

      Dear Paresh,
      You are quite welcome. I’m very glad you found this helpful. Best of luck to you.
      Mike 🙂

  8. The Dark Knight September 6, 2012 at 12:12 pm #

    Mike,
    Do you think that such techniques could be used for CRs esp. numbers and percentages? If yes, it will be great if you could provide some thoughts on it, or would it be an overkill to draw 2X2?

    • Mike MᶜGarry
      Mike September 6, 2012 at 2:46 pm #

      Dark Night:
      That definitely would be overkill. For the exceedingly few questions on which the CR even broaches numerical topics, like percentages, I have never seen it move past what a single equation could handle. I’m very glad that you’re that comfortable with this method, but just use it in the Quant section.
      Mike 🙂

  9. domenico August 29, 2012 at 1:31 pm #

    Hi mike

    really these are the questions might we encounter at top level ???

    Are really challenging. I picked them correct but in two minutes are even more difficult to figure out.

    Gmat is a really beast.

    Thanks for your work Mike.

    • Mike MᶜGarry
      Mike August 29, 2012 at 4:04 pm #

      Domenico: Question #1 is typical of what the GMAT would ask. As I note for Question #2, “it’s debatable whether a set question this difficult even would appear on the GMAT.” It really is way out toward the outer limit of what the GMAT would ever ask. Even so, it’s pretty easy with the DM Method.
      Does that make sense?
      Mike 🙂

      • domenico August 29, 2012 at 4:30 pm #

        Definitely 🙂

        • Mike MᶜGarry
          Mike August 29, 2012 at 6:12 pm #

          Very good. Best of luck to you.
          Mike 🙂

  10. Sourav August 24, 2012 at 10:11 pm #

    After implementing the strategy “this type of question” seems pretty simple. Thanks!

    • Mike MᶜGarry
      Mike August 25, 2012 at 2:30 pm #

      You are quite welcome. Best of luck to you.
      Mike 🙂


Magoosh blog comment policy: To create the best experience for our readers, we will only approve comments that are relevant to the article, general enough to be helpful to other students, concise, and well-written! 😄 Due to the high volume of comments across all of our blogs, we cannot promise that all comments will receive responses from our instructors.

We highly encourage students to help each other out and respond to other students' comments if you can!

If you are a Premium Magoosh student and would like more personalized service from our instructors, you can use the Help tab on the Magoosh dashboard. Thanks!

Leave a Reply

Share5
Tweet
Share
Pin