There is only one sentence in the first half of probability theory: everyone is equal before God. In other words, under the same conditions, the probability of the same event is the same. For example, there are three white balls and five black balls in the box, so the probability of getting a white ball with any ball is 3/8, because the probability of getting each ball is the same. To understand this truth, you need to carefully analyze one thing and divide it into several basic events (that is, events that cannot be divided again). For example, what is the probability of getting a black ball and a white ball? Let's analyze how this happened first. In the first case, you get a black ball first and then a white ball, so the probability of getting the black ball for the first time is 5/8, and the probability of getting the white ball is 3/7 (there are only 7 balls at this time). Then the probability of the first case is 5/8*3/7. In the analysis of the second situation, first get the white ball, then get the black ball ... Understand this truth, there is no problem in passing the exam. Analysis, or analysis!