新gre数学重要考点：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.
