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Of all the students in a certain dormitory, 1/2 are first-ye [#permalink]

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27 Dec 2009, 10:47

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Of all the students in a certain dormitory, 1/2 are first-year students and the rest are second-year students. If 4/5 of the first-year students have not declared a major and if the fraction of second-year students who have declared a major is 3 times the fraction of first-year students who have declared a major, what fraction of all the students in the dormitory are second-year students who have not declared a major?

I think you dropped a fraction when you typed/copied this in. Working backward from the spoiler answer, I'm thinking the second sentence should read as follows? Please confirm.

"If 4/5 of the first-year students have not declared a major ..." _________________

Emily Sledge | Manhattan GMAT Instructor | St. Louis

Of all the students in a certain dormitory, 1/2 are first-year students and the rest are second-year students. If 4/5 of the first-year students have not declared a major and if the fraction of second-year students who have declared a major is 3 times the fraction of first-year students who have declared a major, what fraction of all the students in the dormitory are second-year students who have not declared a major?

\(\frac{4}{5}x\) of the first-year students have not declared a major --> \(\frac{1}{5}x\) declared;

Second-year students who have declared a major is 3 times the fraction of first-year students who have declared a major =\(\frac{3}{5}x\) --> who have not \(\frac{2}{5}x\);

Re: Of all the students in a certain dormitory, 1/2 are first-year student [#permalink]

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12 Aug 2011, 15:23

I think the answer choices you have provided because the answer cannot be greater than one.

Here is an easy way to think about this is by picking "smart" numbers. Let's pick 50 for the first half of students who are 1st years and 50 for the other half who are 2nd year. We know that 4/5 of the 1st years have not declared a major so that gives us 50*4/5=40, so 10 have declared a major. We also know that the number of 2nd year that have not declared a major = 3 times the # of 1st year who have declared a major. This gives us 3*10=30

The question asks for fraction of 2nd years who have not declared a major over the total students. 30 have declared a major out of the 2nd year so 20 have not. Since we have 100 students (50+50), the answer is 20/100 or 1/5.

You can run a search on the web and you will find this question without the correct answer choices.

Re: Of all the students in a certain dormitory, 1/2 are first-ye [#permalink]

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01 Sep 2014, 19:09

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Re: Of all the students in a certain dormitory, 1/2 are first-ye [#permalink]

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28 Apr 2016, 23:06

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 all the students in a certain dormitory, 1/2 are first-year student [#permalink]

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05 May 2016, 21:38

Expert's post

mojorising800 wrote:

Of all the students in a certain dormitory, 1/2 are first-year students and the rest are second-year students. If 4/5 of the first-year students have not declared a major and if the fraction of second-year students who have declared a major is 3 times the fraction of first-year students who have declared a major, what fraction of all the students in the dormitory are second-year students who have not declared a major?

A. 151 B. 51 C. 154 D. 31 E. 52

Say there are in all 100 students. Use a matrix: 50 are first year and 50 are second year students.

........................Major Dec................Major not dec..............Total First Year............................................................................50 Second Yr...........................................................................50 Total

4/5 * 50 = 40 not declared major first year students.

........................Major Dec................Major not dec..............Total First Year................10..................................40....................50 Second Yr...........................................................................50 Total

second-year students who have declared a major is 3 times the fraction of first-year students who have declared a major

........................Major Dec................Major not dec..............Total First Year................10..................................40.....................50 Second Yr...............30..................................20......................50 Total

So second year students who've not declared a major is 20/100 = 1/5

Re: Of all the students in a certain dormitory, 1/2 are first-year student [#permalink]

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27 Jun 2016, 05:51

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This post received KUDOS

The answer choices provided are not correct!

I solved the question using LCM method. The solution is given below:

Assume the total number of stuents to be 30, LCM(2,3,5)

Now, as per the given info, half the students are 1st year. Therefore, there are 15 students each in 1st and 2nd year. We can further calculate the students who have selected majors in 1st year and second year. And, finally we can calculate the fraction for answering the question

Re: Of all the students in a certain dormitory, 1/2 are first-ye [#permalink]

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27 Jun 2016, 06:22

Expert's post

samratpahwa wrote:

The answer choices provided are not correct!

I solved the question using LCM method. The solution is given below:

Assume the total number of stuents to be 30, LCM(2,3,5)

Now, as per the given info, half the students are 1st year. Therefore, there are 15 students each in 1st and 2nd year. We can further calculate the students who have selected majors in 1st year and second year. And, finally we can calculate the fraction for answering the question

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