In general practice, the method of "permutation and combination" is often used to calculate, and p (a) = the number of samples contained in a/the number of samples contained in the population. Given the occurrence of event B, the probability of occurrence of A is called conditional probability, which is denoted as P(A|B). Knowing the significance of the probability of equal possibility events, we use the basic formula of permutation and combination to calculate the probability of some equal possibility events.