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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 ?

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...

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

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

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?