the Fundamental Counting Principle
noun [ S ] uk/ˌfʌn.də.men.təl ˈkaʊn.tɪŋ ˌprɪn.sə.pəl/ us/ˌfʌn.də.men.t̬əl ˈkaʊn.t̬ɪŋ ˌprɪn.sə.pəl/
基本计数原理
the rule that says that if there are x ways in which one thing can be done, and y ways in which another thing can then be done, then there are x × y ways in which the two things can be done one after the other