A student needs to answer at least 3 out of 4 questions correctly in order to pass a test. The questions are chosen at random from a collection of 40 questions, 30 of which the student knows the answer to.
How do I calculate the probability of something like this?
EDIT:
My initial approach
I tried modeling this as an urn consisting of 30 blue and 10 red balls. Then drawing 4 balls from that urn, what would be the probability of getting at least 3 blue balls.
Best Answer
He can get all 4 right. How many ways to choose 4 question from the set of 30 that he knows the answer to?
Or, he can get 3 right and one wrong. How many ways to choose 3 from a set of 30 plus 1 from a set of 10.
All divided by the number or ways to choose 4 questions for the test.