The result of a test is recorded as follows — Boys: Pass 60, Fail 30. Girls: Pass 80, Fail 30. One student is picked at random from those who passed. What is the probability that the student picked is a girl?

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Accepted Answer

4/7. Restrict attention to the passing students: 60 + 80 = 140, of whom 80 are girls. The conditional probability is 80/140 = 4/7. Dividing by the whole group of 200 would wrongly give 2/5.

The result of a test is recorded as follows — Boys: Pass 60, Fail 30. Girls: Pass 80, Fail 30. One student is picked at random from those who passed. What is the probability that the student picked is a girl?

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