Principle of classification and counting

Principle of classification and counting: To do one thing, there are n types of methods. In the first type of method, there are m1 different methods, in the second type of method, there are m2 different methods,..., in the first type of method, there are m2 different methods. There are mn different methods in n types of methods, so there are N=m1 m2... mn different methods to complete this thing.

Principle of classification and counting: To do one thing, there are n types of methods. In the first type of method, there are m1 different methods. In the first type of method, there are m1 different methods. There are m2 different methods in,..., there are mn different methods in the nth method, then there are N=m1 m2...mn different methods to complete this thing.

Example:

Principle of classification and counting: A tour group can take a bus, train, or plane from Nanjing to Shanghai. Assume that there are 3 buses a day, 2 trains a day, and 1 plane a day. How many different ways are there from Nanjing to Shanghai in a day? The answer is 3 2 1=6 kinds

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