Until 1995, the rules for three-digit area codes in the United States were as follows: •The first digit could not be 0 or 1. •The second digit had to be 0 or 1. •The third digit had no such restrictions. In 1995, the restriction on the second digit of area codes was removed. How many area codes are possible after the restriction is removed?
Permutations and Combinations
If there are 5 dishes, they can be relished in any order at a time. In permutation, it should be in a particular order. In combination, the order does not matter. Take 3 letters a, b, and c. The possible ways of pairing any two letters are ab, bc, ac, ba, cb and ca. It is in a particular order. So, this can be called the permutation of a, b, and c. But if the order does not matter then ab is the same as ba. Similarly, bc is the same as cb and ac is the same as ca. Here the list has ab, bc, and ac alone. This can be called the combination of a, b, and c.
Counting Theory
The fundamental counting principle is a rule that is used to count the total number of possible outcomes in a given situation.
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