There are n ways to do something and finish it. The first way has m 1 different ways, the second way has m2 different ways, …, and the nth way has mn different ways, so there are N=m 1+m2+m3+…+mn different ways to accomplish it. Each method can directly achieve the goal.

Multiplication principle

To do one thing, it needs to be divided into n steps. There are m 1 different methods to do the first step, m2 different methods to do the second step, ... and mn different methods to do the n step, so there are n = m1× m2× m3×××××× Mn different methods to do it.