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DS - number of students

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DS - number of students

by oks » Sun Jul 05, 2009 1:46 pm
If each of the students in a certain mathematics class is either a junior or a senior, how
many students are in the class?
(1) If one student is to be chosen at random from the class to attend a conference,
the probability that the student chosen will be a senior is 4/7.
(2) There are 5 more seniors in the class than juniors.

OA is C, but why not A?


Thanks!
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Source: — Data Sufficiency |
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Re: DS - number of students

by ssmiles08 » Sun Jul 05, 2009 4:25 pm
oks wrote:If each of the students in a certain mathematics class is either a junior or a senior, how
many students are in the class?
(1) If one student is to be chosen at random from the class to attend a conference,
the probability that the student chosen will be a senior is 4/7.
(2) There are 5 more seniors in the class than juniors.

OA is C, but why not A?


Thanks!
A is not sufficient b/c choice 1 is giving you a ratio not the actual number.

4x out of 7x students are seniors.

from B we can see that S = 5 + J

we know 4X = S, 3X = J, 7X = T

4x = 3X + 5
x = 5

so the number of seniors is 4x = 4*5 = 20.

(C)
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