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# of the students at a certain school, 50% of the students are

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VP
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of the students at a certain school, 50% of the students are [#permalink]

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15 Mar 2005, 08:15
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of the students at a certain school, 50% of the students are in class a, 30% of the students are in class b, 20% of the students are in class c. of the students in class a, 20% are wearing eyeglasses. of the students in class b, 10% are wearing eyeglasses. of the students c, 10% are wearing eyeglasses. if a student is selected at random, what is the probability that the student is in class b ?
VP
Joined: 13 Jun 2004
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15 Mar 2005, 21:42
I think the question is not complete...because the glasses story is totally useless so I assume that normally it should be a question in realtion with the prob of someone from class b with glasses or without glasses...

maybe I've missed something

if not, my answer is 3/10
Manager
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16 Mar 2005, 22:02
If 30% of the students are in class B, the probability of one of them being picked is clearly 3/10. Is part of the question missing?
VP
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20 Mar 2005, 17:03
christoph please explain or check again your question because I don't get it... please explain the way of thinking
VP
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21 Mar 2005, 03:49
Antmavel wrote:
christoph please explain or check again your question because I don't get it... please explain the way of thinking

possible outcomes: 30% of the students are in class b * of the students in class b, 10% are wearing eyeglasses = 0,1 * 0,3 = 0,03

all outcomes: 50% of the students are in class a * of the students in class a 20% are wearing eyeglasses + 30% of the students are in class b * of the students in class b, 10% are wearing eyeglasses + 20% of the students are in class c * of the students c, 10% are wearing eyeglasses = 0,15

is it clear ?
VP
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Re: PS - Bayes Theorem [#permalink]

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21 Mar 2005, 03:56
christoph wrote:
if a student is selected at random, what is the probability that the student is in class b ?

I think there is a mistake in the first post. Actually the answer to this question should be :
total outcome : all students
favorable outcome : all students in class b

however, the solution you've given us is
total outcome : all students with glasses
favorable outcome : all students with glasses in class b

Is it clear ?
Senior Manager
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21 Mar 2005, 03:58
Christoph, I still don't get it.

The question asks "what is the probability that the student is in class b": How can the possible outcomes depend on the number of students wearing glasses or not?

Is there something missing in the question or something which I have misinterpreted?

Thanks.
VP
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21 Mar 2005, 04:10
hm? you guys are right. i think the question is wrong. it should be: "what is the probability that the student in class b is wearing glasses?" sorry !
Manager
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23 Mar 2005, 04:42
It would be 30%*10%= 3%

Another interesting way to look at it would be what percent of students do not wear eyeglasses.

(50%*20%)+(30%*10%)+(20%*10%)= 10%+3%+2%= 15%
[#permalink] 23 Mar 2005, 04:42
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# of the students at a certain school, 50% of the students are

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