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According to the record of the registrar's office at a state university, 35% of the students are freshman, 25% are sophomore, 16% are junior and the rest are senior. Among the freshmen, sophomores, juniors and seniors, the portion of students who live in the dormitory are, respectively, 80%, 60%, 30% and 20%. If a randomly chosen student does not live in the Dormitory, what is the probability that he is a sophmore ?

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  • $\begingroup$ And your question about the problem is..? $\endgroup$ – Slug Pue Feb 10 '17 at 17:11
  • $\begingroup$ what is the answer? $\endgroup$ – Gag Gago Feb 10 '17 at 17:14
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    $\begingroup$ meta.math.stackexchange.com/questions/9959/… $\endgroup$ – Slug Pue Feb 10 '17 at 17:17
  • $\begingroup$ thank you for making fun of my name. $\endgroup$ – Gag Gago Feb 10 '17 at 17:19
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    $\begingroup$ This is a question about conditional probability. Where are you getting stuck? $\endgroup$ – John Feb 10 '17 at 17:36
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An outline leading to the solution:

According to the definition of conditional probability, $P(So|D^c) = P(So \cap D^c)/P(D^c).$

For the numerator: $P(So \cap D^c) = P(So)P(D^c|So) = .25(1-.6) = 0.1.$

For the denominator: $$P(D^c) = P(F \cap D^c)+P(So \cap D^c)+P(J \cap D^c)+P(Sr \cap D^c).$$ This is an example of the Law of Total Probability.

We have already found one of these four probabilities. Find the other three in a similar way, and add.

Then do the division for the final answer.

This is an example of Bayes' Theorem. (Hence my edit to the title of your question.)

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  • $\begingroup$ Still there? Can you find $P(F \cap D^c)?$ Back in a few hours. $\endgroup$ – BruceET Feb 11 '17 at 2:00

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