The combination formula in the probability formula is: c(n, m)=n! /[(n-m)！ *m！ ], which is equal to the product of m natural numbers decreasing continuously from n divided by the product of m natural numbers increasing continuously from 1.

So the first formula equals 4, the second formula equals 120, and the third formula equals 2. The calculation process is as follows:

Extended data

Classical probability, also called traditional probability, was defined by French mathematician Laplace? (Laplace). If a random test contains finite unit events and the probability of each unit event is equal, then this random test is called Laplace test, and the probability model under this condition is called classical probability.

Under this model, all possible results of random experiments are limited, and the probability of each basic result is the same. Classical probability is the most intuitive and simple model in probability theory, and many operational rules of probability are obtained for the first time under this model.