Permutation & Combination

Combination & Permutation deals with arrangement of thing. If the order doesn't matter, then it is called Combination. If the order does matter, then it is a Permutation.

In other words, Permutation is an ordered Combination.

Permutation 

There are basically two types of permutation:

1. When repetition is allowed
2. No repetition

1. Permutations with Repetition
To choose r things from n when repetition is allowed, the permutations are:

n × n × ... (r times) = n^r

(Because there are n possibilities for the first choice, then there are n possibilites for the second choice, and so on.)

Combinations

Number of ways objects can be selected from a group.
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