OF the 800 students at a certain college, 250 students live : GMAT Data Sufficiency (DS)
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# OF the 800 students at a certain college, 250 students live

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OF the 800 students at a certain college, 250 students live [#permalink]

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31 Jan 2008, 18:53
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OF the 800 students at a certain college, 250 students live on campus and are more than 20 years old. How many of the 800 students live on campus and are 20 years old or less?

1. 640 students at the college are more than 20 years old.
2. 60 students at the college are 20 years old or less and live off campus.
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Re: OF the 800 students at a certain college, 250 students live [#permalink]

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31 Jan 2008, 20:30
blog wrote:
OF the 800 students at a certain college, 250 students live on campus and are more than 20 years old. How many of the 800 students live on campus and are 20 years old or less?

1. 640 students at the college are more than 20 years old.
2. 60 students at the college are 20 years old or less and live off campus.

C. Use a double set matrix

We find that the number of On campus under 20 yrs old =100.
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Re: OF the 800 students at a certain college, 250 students live [#permalink]

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31 Jan 2008, 20:49
C

from stat 1 all you can tell is the number of 20 plus yr olds who live off campus

from stat 2 all you can tell is the number of less than 20s who live off campus

together, you can find the number of total students who live off campus, and therefore, the number that live on campus . as you already know the number who live on campus that are 20 plus, the difference between the total and that number will be the number of less than 20s who live on campus.
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Re: OF the 800 students at a certain college, 250 students live [#permalink]

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03 Jan 2015, 20:03
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OF the 800 students at a certain college, 250 students live [#permalink]

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04 Jan 2015, 23:38
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blog wrote:
OF the 800 students at a certain college, 250 students live on campus and are more than 20 years old. How many of the 800 students live on campus and are 20 years old or less?

1. 640 students at the college are more than 20 years old.
2. 60 students at the college are 20 years old or less and live off campus.

This question involves a lot of data points, and therefore can quickly get overwhelming!

The key to solving such questions is to represent the given information visually.

One good visual representation for the given info is the Double set matrix as mentioned by GMATBLACKBELT above.

Another representation is as attached. In our concept files, we call this a TREE STRUCTURE.

The question asks us about the number of students who live on campus and are <= 20 years. Let the number of such students = x

Therefore, total number of students who live ON campus = 250 + x

Total number of students who live OFF campus = 800 - (250 + x) = 550 - x

Let the number of students who live OFF campus and are >20 years = y
This means, the number of students who live OFF campus and are <= 20 years = (550 - x) - y

(Refer to Attached Picture for visual representation of this info on the Tree Structure)

We are now ready to assess if the information given in Statements 1 and 2 is sufficient to determine the value of x.

Per Statement 1,
250 + y = 640

This equation gets us the value of y. We need the value of x. Clearly, Statement 1 alone is not sufficient.

Per Statement 2,
550 - x - y = 60

This is a linear equation in 2 variables. Since we have only equation, we will not be able to determine a unique value of x. Therefore, Statement 2 alone is not sufficient either.

Combining Statements 1 and 2:

We get two linear equations in two variables. Solving both these equations together, we WILL be able to get a unique value of x.

Therefore, the correct answer choice is C - Both the statements together are sufficient to answer the question.

The Important Takeaway from this discussion: VISUAL Representation of the given data is the key to simplifying questions involving two or more sets.
Attachments

Picture2.png [ 24.83 KiB | Viewed 3477 times ]

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Re: OF the 800 students at a certain college, 250 students live [#permalink]

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24 Jun 2015, 13:02
blog wrote:
OF the 800 students at a certain college, 250 students live on campus and are more than 20 years old. How many of the 800 students live on campus and are 20 years old or less?

1. 640 students at the college are more than 20 years old.
2. 60 students at the college are 20 years old or less and live off campus.

Set up a 3x3 table to solve this:

Attachment:

Table.jpg [ 25.25 KiB | Viewed 2773 times ]

Target value is marked with question mark.
Grey values are values derived from the question stem.
Orange value is derived from statement 1. With this you can calculate "Off Campus / > 20Yrs" = 390. However this does not lead us to the question mark. > IS
Blue Value is derived from statement 2. With this on its own, you cannot find "?". > IS

Together you will be able to find "?" in the table. 640 + X = 800 and X - 60 = ?
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Re: OF the 800 students at a certain college, 250 students live [#permalink]

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21 Jan 2017, 22:14
Hello from the GMAT Club BumpBot!

Thanks to another GMAT Club member, I have just discovered this valuable topic, yet it had no discussion for over a year. I am now bumping it up - doing my job. I think you may find it valuable (esp those replies with Kudos).

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Re: OF the 800 students at a certain college, 250 students live   [#permalink] 21 Jan 2017, 22:14
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# OF the 800 students at a certain college, 250 students live

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