Lesson: Double Matrix Method

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Euro's picture

Another method shows the matrix a 3X3 to leave room for the totals, in the 2X2 you write out the number anyway.
gmat-admin's picture

That method is exactly the same. The only difference is a small change in how we show the totals for each row and column.

For the motel ques below, the 2/3rd statement is a bit confusing. Read it as 2/3 of rented, not 2/3 of AC rooms. How can I avoid mistakes like this - the question writing is a bit vague if you ask me

One night a certain motel rented 3/4 th of its rooms, including 2/3rd of its air-conditioned rooms. If 3/5th of its rooms were air-conditioned, what percent of the rooms that were not rented were air-conditioned?
gmat-admin's picture

You're referring to this question: http://www.beatthegmat.com/motel-t271938.html

This is an official GMAT question, and I think the wording is fine. We're told that 2/3 of the air-conditioned rooms were rented, so I'm not sure what could be changed to make it more readable.

Hi, Brent! I used the double Matrix Method for the following question. But I got different result than the answer choice. I got 34%. After compared with the explanation, I think the major confusion that I have is how to interpret "75% of the interns receive fewer than 6 hours of sleep and report feeling tired during their shifts". This to me means out of all interns who received fewer than 6 hours of sleep, there are 75% report feeling tired during their shifts. If I used 100 as total intern number, the grid of less than 6 hours and tired intern should be 0.75 X 80 = 60. But the answer states that there are 75 people in this grid. Could you please help me out?

At a certain hospital, 75% of the interns receive fewer than 6 hours of sleep and report feeling tired during their shifts. At the same time, 70% of the interns who receive 6 or more hours of sleep report no feelings of tiredness. If 80% of the interns receive fewer than 6 hours of sleep, what percent of the interns report no feelings of tiredness during their shifts?

A) 6
B) 14
C) 19
D) 20
E) 81

Answer C
gmat-admin's picture

Hi Jingyjess,

The key word here is AND: "At a certain hospital, 75% of the interns receive fewer than 6 hours of sleep AND report feeling tired during their shifts."

If the test-makers wanted to convey your interpretation, they'd write "OF THE interns who received fewer than 6 hours of sleep, 75% report feeling tired during their shift." (Notice that's there's no AND here. Instead, we have OF THE)

Does that help?

Cheers,
Brent

Thank you for the feedback!I understand where you are coming from. I just still need take some time to digest and prepare better strategy for tricky wordings like this in the future!

Hey Brent,
Please help me with the following question posted on Beat the GMAT. I am extremely confused wrt using the double matrix method for this.

At least 100 students at a certain high school study Japanese. If 4 percent of the students at the school who study French also study Japanese, do more students at the school study French than Japanese?
(1) 16 students at the school study both French and Japanese.
(2) 10 percent of the students at the school who study Japanese also study French.

Thanks in advance,
Aashay
gmat-admin's picture

Hi Aashay,

I'm happy to help.
You'll find my solution here: http://www.beatthegmat.com/at-least-100-students-at-a-certain-high-schoo...

Cheers,
Brent

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