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Every student of a certain school must take one and only one [#permalink]

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19 Feb 2012, 07:30

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Every student of a certain school must take one and only one elective course. In last year, 1/2 of the students took biology as an elective, 1/3 of the students took chemistry as an elective, and all of the other students took physics. In this year, 1/3 of the students who took biology and 1/4 of the students who took chemistry left school, other students did not leave, and no fresh student come in. What fraction of all students took biology and took chemistry?

Biology students left = (1/2)/3 = 1/6. So remaining = 1/2 - 1/6 = 1/3 ------------- (i) Chemistry students left = (1/3)/4 = 1/12. So remaining = 1/3 - 1/12 = 1/4 --------------- (ii)

Total Biology & Chemistry students left = (i) + (ii) = 7/12 Total all students left = 7/12 + 1/6 = 9/12 = 3/4

Re: Every student of a certain school must take one and only one [#permalink]

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31 Jan 2014, 07:08

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Chembeti wrote:

Every student of a certain school must take one and only one elective course. In last year, 1/2 of the students took biology as an elective, 1/3 of the students took chemistry as an elective, and all of the other students took physics. In this year, 1/3 of the students who took biology and 1/4 of the students who took chemistry left school, other students did not leave, and no fresh student come in. What fraction of all students took biology and took chemistry?

A. 7/9 B. 6/7 C. 5/7 D. 4/9 E. 2/5

The answer is A but the wording is quite poor. The question should ask what fraction of the remaining students took biology or chemistry

That will make the question more clear

Anyways if you pick 120 as a Smart Numbers you get 70/90 = 7/9 (A)

Re: Every student of a certain school must take one and only one [#permalink]

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31 Jan 2014, 13:15

Chembeti wrote:

Every student of a certain school must take one and only one elective course. In last year, 1/2 of the students took biology as an elective, 1/3 of the students took chemistry as an elective, and all of the other students took physics. In this year, 1/3 of the students who took biology and 1/4 of the students who took chemistry left school, other students did not leave, and no fresh student come in. What fraction of all students took biology and took chemistry?

A. 7/9 B. 6/7 C. 5/7 D. 4/9 E. 2/5

This is kind of confusing. Shouldn't the question be asking what fraction of students took either biology or chemistry?

Every student of a certain school must take one and only one [#permalink]

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24 Dec 2014, 11:14

What fraction of all students took biology and took chemistry? OR What fraction of the remaining students took biology and took chemistry?

For the former correct answer will be 5/6 [(1/2+1/3)/(1)] and for the later correct answer will be 7/9[(1/2*2/3+1/3*3/4)/(1/3+1/4+1/6)].

It took me 7 minutes to understand that too after matching the answers that what exactly the question is ??

Chembeti wrote:

Every student of a certain school must take one and only one elective course. In last year, 1/2 of the students took biology as an elective, 1/3 of the students took chemistry as an elective, and all of the other students took physics. In this year, 1/3 of the students who took biology and 1/4 of the students who took chemistry left school, other students did not leave, and no fresh student come in. What fraction of all students took biology and took chemistry?

A. 7/9 B. 6/7 C. 5/7 D. 4/9 E. 2/5

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Re: Every student of a certain school must take one and only one [#permalink]

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20 Feb 2016, 11:31

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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2nd year: Students who left from Biology(60) and Chemistry(40): 1/3*60 + 1/4*40 = 30 Students left in school taking Biology and Chemistry = 60 + 40 - 30 = 70 Students left in school = 120 - 30 = 90 Fraction of students taking Biology and Chemistry = 70/90 = 7/9