Probability problem of classical binomial distribution. Special note: the author of this question has set something that is easy to confuse thinking, that is, the extension dialing rate is 5%, and the call rate is guaranteed to be 95%. Two completely different quantities and numerical settings seem to have a great relationship, and beginners may be confused here. See the big picture for details:? (Note: It is not clear to look at the big picture directly. You need to click and drag the big picture to a new page to view it. There is a clerical error in the picture, instead of "n is much greater than q", it should be changed to "n q is much greater than 1")