Originally Posted by

**xian791** Ello all,

I'm doing the practice test for a stats class and I'm stuck on a question.

A special test is given to people complaining of severe headaches. If someone has a brain tumor, the test is positive with a probability of .85(85%) If they have no tumor, the test is positive with a probability of .1(10%) Only 4%(.04) of those tested actually have brain tumors.

Whats is the probability that someone who tested positive has a brain tumor?

The answer is 0.262, but I have no idea how to get to that answer.

At first they ask "what is the probability someone random selected from those who take the test has a brain tumor AND test positive?"

For that I multiplied .85 by .04 and got the correct answer of 0.034

Thanks,

Shawn