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# 2.18: Mean

Difficulty Level: At Grade Created by: CK-12
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Have you ever heard of the word "average" in math? Have you ever tried to figure out the average or mean of a set of numbers?

Tania and Alex are continuing to plan for next year’s garden. Today, Tania has decided to complete a harvesting review of carrots. She wants to use the number of carrots that were picked each week to make some conclusions about carrot growth. First, she wants to figure out the mean or the average number of carrots that were picked.

Here is Tania’s data about the number of carrots picked each week over nine weeks of harvest.

2, 8, 8, 14, 9, 12, 14, 20, 19, 14

This is a total of 120 carrots-the number of carrots that we saw in the last Concept.

Use what you will learn in this Concept to help Tania.

### Guidance

The first way of analyzing data that we are going to learn about is called the mean. A more common name for the mean of a set of data is to call it the average. In other words, the mean is the average of the set of data.

An average lets us combine the numbers in the data set into one number that best represents the whole set. First let’s see how to find the mean, and then we’ll learn more about how to use it to interpret data.

There are two steps to finding the mean.

1. We add up all of the numbers in the data set.
2. We divide the total by the number of numbers in the set.

10, 7, 3, 8, 2

First, we need to add all the numbers together.

10 + 7 + 3 + 8 + 2 = 30

Now we divide the total, 30, by the number of items in the set. There are 5 numbers in the set, so we divide 30 by 5.

30 $\div$ 5 $=$ 6

The mean , or average , of the set is 6.

Next, let’s see how finding the mean helps us interpret data.

Suppose we want to know how tall plants grow when we add a certain nutrient to the water. The data below shows the height in inches of 10 plants grown with the nutrient-rich water.

9, 10, 7, 3, 11, 9, 8, 11, 7, 10

Let’s find the mean. Add up all of the numbers first.

9 + 10 + 7 + 3 + 11 + 9 + 8 + 11 + 7 + 10 = 85

Now we divide by the number of items in the data set. There are 10 plants, so we get the following answer.

85 $\div$ 10 $=$ 8.5

The mean height of the plants is 8.5 inches. This gives us a nice estimate of how tall a plant might grow with the nutrient-rich water. Let’s see where the mean falls in relation to the other numbers in the set. If we reorder the numbers, we get

3, 7, 7, 8, 9, 9, 10, 10, 11, 11

The minimum of the set is 3 and the maximum is 11. Take a good look at all of the numbers in the set. Here are some conclusions that we can draw from this data.

• Only 3 stands out by itself at one end of the data set. Since it is much smaller than the other numbers, we might assume that this plant didn’t grow very well for some reason.

We can make a prediction based on this.

• Perhaps of the 10 plants it got the least light, or maybe its roots were damaged.

The mean helps even out any unusual results such as the height of this one plant.

Now let's practice. Find the mean for each set of data.

#### Example A

3, 4, 5, 6, 2, 5, 6, 12, 2

Solution: 5

#### Example B

22, 11, 33, 44, 66, 76, 88, 86, 4

Solution: 47.7 or round up to 48

#### Example C

37, 123, 234, 567, 321, 909, 909, 900

Solution: 500

Here is Tania’s data about the number of carrots picked each week over nine weeks of harvest.

2, 8, 8, 14, 9, 12, 14, 20, 19, 14

This is a total of 120 carrots-the number of carrots that we saw from the last section.

First, we can underline all of the important information. Next, let’s find the mean

What is the average amount of carrots that were picked overall?

To answer this question, we add up the values in the data set and divide by the number of values in the data set.

$2 + 8 + 8 + 14 + 9 + 12 + 14 + 20 + 19 + 14 &= 120\\120 \div 10 &= 12$

The mean or average is 12.

### Vocabulary

Here are the vocabulary words in this Concept.

Mean
the average of a set of numbers. The mean gives us a good overall assessment of a set of data.
Maximum
the greatest score in a data set
Minimum
the smallest score in a data set

### Guided Practice

Here is one for you to try on your own.

Jacob has the following quiz scores.

78, 90, 83, 88, 67, 90, 84, 69

Given these scores, what is his average for the quarter?

To begin, add up all of the scores.

$78 + 90 + 83 + 88 + 67 + 90 + 84 + 69 = 649$

Next, we divide by the number of scores.

$649 \div 8 = 81.1$

Jacob's average is an 81.

### Video Review

Here is a video for review.

### Practice

Directions: Find the mean for each set of data. You may round to the nearest tenth when necessary.

1. 4, 5, 4, 5, 3, 3

2. 6, 7, 8, 3, 2, 4

3. 11, 10, 9, 13, 14, 16

4. 21, 23, 25, 22, 22, 27

5. 27, 29, 29, 32, 30, 32, 31

6. 34, 35, 34, 37, 38, 39, 39

7. 43, 44, 43, 46, 39, 50

8. 122, 100, 134, 156, 144, 110

9. 224, 222, 220, 222, 224, 224

10. 540, 542, 544, 550, 548, 547

11. 762, 890, 900, 789, 780, 645, 700

12. 300, 400, 342, 345, 403, 302

13. 200, 199, 203, 255, 245, 230, 211

14. 1009, 1000, 1200, 1209, 1208, 1217

15. 2300, 2456, 2341, 2400, 2541, 2321

Oct 29, 2012

Dec 29, 2014