The total probability theorem and the Bayes’ theorem are important and interesting contents taught in probability at the university. In the first part of this article, we use the total probability theorem and the Bayes’ theorem to solve some well-known elementary probability problems, such as the problem of fairness in the lucky draw, the Monty Hall problem. As for the lucky draw problem, we present a tighter and more general solution than the known solutions. At the end of the article, we will introduce some applications of the Bayesian theorem in medicine and search and rescue operations. Hopefully this article will bring some useful things for those who are starting to teach or study probability.