GRE数学重要知识点：Combinations of Choices

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　　Combinations of Choices

　　If there are n choices for one type and m choices for second type, then number of combinations for both types is the product of choices:n*m.

　　Example:

　　If there are 3 kinds of breads to choose from and 4 kinds of meats, then we can make 3*4=12 different sandwiches.

　　If an alphabet has 26 letters, then the number of possible 2 letter word is 26*26=676.All of them may not be vaild words.

　　Remember:

　　Do not add the choices to get the total combinations.

　　The product rule can be applied to more than two types of choices: if there are 3 kinds of breads, 4 kinds of meats, and 6 kinds of cheeses, then we can make 3*4*6=72 different sandwiches.