## Permutations and Combinations

### 2.5 STEP THREE: The Labeling Principle

Lesson materials located below the video overview.

 In how many ways can we arrange the letters of the Swedish pop group name  ABBA?

Answer: $$\dfrac{4!}{2!2!} = \dfrac{24}{4} = 6$$.

 In how many ways can we arrange the letters of AABBBBA?

Answer: $$\dfrac{7!}{3!4!}$$.

 In how many ways can we arrange the letters of AAABBBBCCCCCC?

Answer: $$\dfrac{13!}{3!4!6!}$$.

Let’s look at this third problem and phrase it in a different way:

 Mean Mr. Muckins has a class of 13 students. He has decided to randomly assign the grade of A to three students, the grade of B to four students, and the grade of C to six students. In how many ways could he assign these labels?

Answer: Let’s imagine all thirteen students are in a line.

Here’s one way he can assign labels:

Here’s another way:

and so on.

We see that this labeling problem is just the same problem as rearranging letters. The answer must be $$\dfrac{13!}{3!4!6!}$$.

 Of 10 people in an office 4 are needed for a committee. How many ways?

Answer: Imagine the 10 people standing in a line. We need to give out labels. Four people will be called “ON” and six people will be called “LUCKY.” Here is one way to assign those labels:

We see that this is just a word arrangement problem. The answer is:

$$\dfrac{10!}{4!6!}=\dfrac{10 \cdot 9 \cdot 8 \cdot 7}{4 \cdot 3 \cdot 2 \cdot 1} = 10 \cdot 3 \cdot 7 = 210$$.

In general, we have …

 THE LABELING PRINCIPLEEach of $$N$$ distinct objects is to be given a label. If $$a$$ of them are to have label “1,” $$b$$ of them to have label “2,” and so on, then the total number of ways to assign labels is:  $$\dfrac{N!}{a!b! \cdots z!}$$.

That’s it. We are just arranging letters, with the letters being the names of the labels.

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