Suppose a certain exam is classified as either difficult (with probability 90/92) or fair (with probability 2/92). Exams are taken one after the other. What is the probability that at least 4 difficult exams will occur before the first fair one?
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It is not clear in your question whether you are expecting to get at least one fair exam. If you want so you have to give us the number of exams you are taking.
Otherwise, the probability you are looking for is exactly the probability that the first 4 exams are difficult, which is $\left(\dfrac{45}{46}\right)^4$.
fonfonx
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ok then my answer seems to be correct since $(45/46)^4=0.9158...$ – fonfonx Jun 23 '17 at 20:04