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Permutation: Definition, Formula & Examples

Permutation: Definition, Formula & Examples
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  • 0:02 Introduction to Permutations
  • 0:17 Involving Repetitions
  • 1:19 Involving No Repetitions
  • 3:10 Any Sized Group of Objects
  • 4:31 Examples
  • 6:15 Lesson Summary
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Lesson Transcript
Instructor: Norair Sarkissian
In this lesson, we will examine a mathematical method of calculating the number of ways in which we can order a group of distinct objects. This involves the use of a mathematical concept known as a permutation.

Introduction to Permutations

A permutation is an arrangement or ordering of a number of distinct objects. For example, the words 'top' and 'pot' represent two different permutations (or arrangements) of the same three letters.

Permutations Involving Repetitions

Let's say that we are taking a test, which consists of five multiple choice questions. There are four different answers (a through d) to choose from for each question. What is the total number of ways in which we could answer the five questions on the test?

One way of thinking about this problem is as follows: let's focus on just the first question on the test. How many choices do we have? The answer to this question is four, since we can pick any answer from a to d. Then we ask the same question for each successive question, and clearly, there are four ways of answering each of them. Therefore, the total number of ways for filling out the test is the product of:

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Since we have the same number of choices for each question, the same scenario is repeated for each question on the test. We can generalize our calculations and state that if there are n choices and r repetitions, then the number of permutations or arrangements is given by:

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Permutations Involving No Repetitions

Now let's consider a somewhat different situation. Let's assume that we have four cell phones of the same make and model, but each one is a different color, so they are distinct objects. We buy four distinct cases for them. How many ways are there for us to place the four different cell phones in the four cases?

We can list all the possible arrangements and then count them:

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We can see that there are 24 unique permutations. We would like to know if it is possible to calculate the total number of permutations without listing all the arrangements and then counting them. It turns out that it's pretty easy to do this if we use a systematic approach to the problem.

Let's start with the first cell phone case: how many choices do we have? The answer is four, since any of the four cell phones could be placed in the first case. When we ask the same question about the second case, we realize that we no longer have four choices, because we already placed a cell phone in the first case, and we now have three cell phones remaining. This means that we have three choices when placing a phone in the second case.

As for the third case, we only have two phones remaining, or two options left. And when we get to the fourth case, we only have a single phone to place in it, so there is only one choice. Thus, the number of ways to place four different cell phones in four different cases is (4)(3)(2)(1) = 24.

Comparing this scenario with the previous one involving the multiple choice test, we can see that there are no repetitions here, since once an object is selected (such as a cell phone placed in a case), it's no longer available for selection. When no repetitions are involved, the number of permutations of n distinct objects is given by n!, where n! is the factorial of the integer n.

Any Sized Groups Of Objects

This scenario involves no repetitions, but it's different from the previous situation. For example, starting with the same four cell phones of different colors, how many arrangements of two phones can we form? In mathematical language, we describe this situation as the number of permutations of 4 objects, taken two at a time.

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