Prealgebra 2e — Original English

Averages and Probability

One application of decimals that arises often is finding the average of a set of numbers. What do you think of when you hear the word average? Is it your grade point average, the average rent for an apartment in your city, the batting average of a player on your favorite baseball team? The average is a typical value in a set of numerical data. Calculating an average sometimes involves working with decimal numbers. In this section, we will look at three different ways to calculate an average.

Calculate the Mean of a Set of Numbers

The mean is often called the arithmetic average. It is computed by dividing the sum of the values by the number of values. Students want to know the mean of their test scores. Climatologists report that the mean temperature has, or has not, changed. City planners are interested in the mean household size.

Suppose Ethan’s first three test scores were 85,88,and94. To find the mean score, he would add them and divide by 3.

85+88+943267389

His mean test score is 89 points.

Find the mean of the numbers 8,12,15,9,and6.

Solution

Solution

Detailed steps and mathematical expressions for calculating the mean of a numerical data set.
Write the formula for the mean: mean=sum of all the numbersn
Write the sum of the numbers in the numerator. mean=8+12+15+9+6n
Count how many numbers are in the set. There are 5 numbers in the set, so n=5. mean=8+12+15+9+65
Add the numbers in the numerator. mean=505
Then divide. mean=10
Check to see that the mean is 'typical': 10 is neither less than 6 nor greater than 15. The mean is 10.

The ages of the members of a family who got together for a birthday celebration were 16,26,53,56,65,70,93,and97 years. Find the mean age.

Solution

Solution

This table provides a step-by-step example of how to calculate the arithmetic mean of a set of numbers.
Write the formula for the mean: mean=sum of all the numbersn
Write the sum of the numbers in the numerator. mean=16+26+53+56+65+70+93+97n
Count how many numbers are in the set. Call this n and write it in the denominator. mean=16+26+53+56+65+70+93+978
Simplify the fraction. mean=4768
mean=59.5

Is 59.5 ‘typical’? Yes, it is neither less than 16 nor greater than 97. The mean age is 59.5 years.

Did you notice that in the last example, while all the numbers were whole numbers, the mean was 59.5, a number with one decimal place? It is customary to report the mean to one more decimal place than the original numbers. In the next example, all the numbers represent money, and it will make sense to report the mean in dollars and cents.

For the past four months, Daisy’s cell phone bills were $42.75,$50.12,$41.54,$48.15. Find the mean cost of Daisy’s cell phone bills.

Solution

Solution

Step-by-step calculation of the mean, illustrating each procedure with its corresponding mathematical expression.
Write the formula for the mean. mean=sum of all the numbersn
Count how many numbers are in the set. Call this n and write it in the denominator. mean=sum of all the numbers4
Write the sum of all the numbers in the numerator. mean=42.75+50.12+41.54+48.154
Simplify the fraction. mean=182.564
mean=45.64

Does $45.64 seem ‘typical’ of this set of numbers? Yes, it is neither less than $41.54 nor greater than $50.12.

The mean cost of her cell phone bill was $45.64

Find the Median of a Set of Numbers

When Ann, Bianca, Dora, Eve, and Francine sing together on stage, they line up in order of their heights. Their heights, in inches, are shown in Table 4.

Ann Bianca Dora Eve Francine
59 60 65 68 70

Dora is in the middle of the group. Her height, 65, is the median of the girls’ heights. Half of the heights are less than or equal to Dora’s height, and half are greater than or equal. The median is the middle value.

The numbers 59, 60, 65, 68, and 70 are listed. 59 and 60 have a brace beneath them and in red are labeled “2 below.” 68 and 70 have a brace beneath them and in red are labeled “2 above.” 65 has an arrow pointing to it and is labeled as the median.

What if Carmen, the pianist, joins the singing group on stage? Carmen is 62 inches tall, so she fits in the height order between Bianca and Dora. Now the data set looks like this:

59,60,62,65,68,70

There is no single middle value. The heights of the six girls can be divided into two equal parts.

The numbers 59, 60, and 62 are listed, followed by a blank space, then 65, 68, and 70.

Statisticians have agreed that in cases like this the median is the mean of the two values closest to the middle. So the median is the mean of 62and65,62+652. The median height is 63.5 inches.

The numbers 9, 11, 12, 13, 15, 18, and 19 are listed. 9, 11, and 12 have a brace beneath them and are labeled “3 below.” 15, 18, and 19 have a brace beneath them and are labeled “3 above.” 13 has an arrow pointing to it and is labeled as the median.

Notice that when the number of girls was 5, the median was the third height, but when the number of girls was 6, the median was the mean of the third and fourth heights. In general, when the number of values is odd, the median will be the one value in the middle, but when the number is even, the median is the mean of the two middle values.

Find the median of 12,13,19,9,11,15,and18.

Solution

Solution

List the numbers in order from smallest to largest. 9, 11, 12, 13, 15, 18, 19
Count how many numbers are in the set. Call this n. n=7
Is n odd or even? odd
The median is the middle value. The image shows the median, 13, as the central value in the ordered dataset (9, 11, 12, 13, 15, 18, 19), with an equal count of 3 values below and 3 values above it.
The middle is the number in the 4th position. So the median of the data is 13.

Kristen received the following scores on her weekly math quizzes:

83,79,85,86,92,100,76,90,88,and64. Find her median score.

Solution

Solution

Find the median of 83, 79, 85, 86, 92, 100, 76, 90, 88, and 64.
List the numbers in order from smallest to largest. 64, 76, 79, 83, 85, 86, 88, 90, 92, 100
Count the number of data values in the set. Call this n. n=10
Is n odd or even? even
The median is the mean of the two middle values, the 5th and 6th numbers. An ordered set of 10 numbers (64, 76, 79, 83, 85, 86, 88, 90, 92, 100) is shown, divided into two groups of 5, with the middle numbers 85 and 86 highlighted to illustrate finding the median.
Find the mean of 85 and 86. mean=85+862
mean=85.5
Kristen's median score is 85.5.

Identify the Mode of a Set of Numbers

The average is one number in a set of numbers that is somehow typical of the whole set of numbers. The mean and median are both often called the average. Yes, it can be confusing when the word average refers to two different numbers, the mean and the median! In fact, there is a third number that is also an average. This average is the mode. The mode of a set of numbers is the number that occurs the most. The frequency, is the number of times a number occurs. So the mode of a set of numbers is the number with the highest frequency.

Suppose Jolene kept track of the number of miles she ran since the start of the month, as shown in Figure 1.

An image of a calendar is shown. On Thursday the first, labeled New Year's Day, is written 2 mi. On Saturday the third is written 15 mi. On the 4th, 8 mi. On the 6th, 3 mi. On the 7th, 8 mi. On the 9th, 5 mi. On the 10th, 8 mi.

If we list the numbers in order it is easier to identify the one with the highest frequency.

2,3,5,8,8,8,15

Jolene ran 8 miles three times, and every other distance is listed only once. So the mode of the data is 8 miles.

The ages of students in a college math class are listed below. Identify the mode.

18,18,18,18,19,19,19,20,20,20,20,20,20,20,21,21,22,22,22,22,22,23,24,24,25,29,30,40,44

Solution

Solution

The ages are already listed in order. We will make a table of frequencies to help identify the age with the highest frequency.

A table is shown with 2 rows. The first row is labeled 'Age' and lists the values: 18, 19, 20, 21, 22, 23, 24, 25, 29, 30, 40, and 44. The second row is labeled 'Frequency' and lists the values: 4, 3, 7, 2, 5, 1, 2, 1, 1, 1, 1, and 1.

Now look for the highest frequency. The highest frequency is 7, which corresponds to the age 20. So the mode of the ages in this class is 20 years.

The data lists the heights (in inches) of students in a statistics class. Identify the mode.

56 61 63 64 65 66 67 67
60 62 63 64 65 66 67 70
60 63 63 64 66 66 67 74
61 63 64 65 66 67 67
Solution

Solution

List each number with its frequency.

A table is shown with 2 rows. The first row is labeled “Number” and lists the values: 56, 60, 61, 62, 63, 64, 65, 66, 67, 70, and 74. The second row is labeled “Frequency” and lists the values: 1, 2, 2, 1, 5, 4, 3, 5, 6, 1, and 1.

Now look for the highest frequency. The highest frequency is 6, which corresponds to the height 67 inches. So the mode of this set of heights is 67 inches.

Some data sets do not have a mode because no value appears more than any other. And some data sets have more than one mode. In a given set, if two or more data values have the same highest frequency, we say they are all modes.

Use the Basic Definition of Probability

The probability of an event tells us how likely that event is to occur. We usually write probabilities as fractions or decimals.

For example, picture a fruit bowl that contains five pieces of fruit - three bananas and two apples.

If you want to choose one piece of fruit to eat for a snack and don’t care what it is, there is a 35 probability you will choose a banana, because there are three bananas out of the total of five pieces of fruit. The probability of an event is the number of favorable outcomes divided by the total number of outcomes.

Two equations are shown. The top equation says the probability of an event equals the number of favorable outcomes over the total number of outcomes. The bottom equation says the probability of choosing a banana equals 3 over 5. There is a blue arrow pointing to the 3 with the text, 'There are 3 bananas.' There is a blue arrow pointing to the 5 with the text, 'There are 5 pieces of fruit.'

Converting the fraction 35 to a decimal, we would say there is a 0.6 probability of choosing a banana.

Probability of choosing a banana=35Probability of choosing a banana=0.6

This basic definition of probability assumes that all the outcomes are equally likely to occur. If you study probabilities in a later math class, you’ll learn about several other ways to calculate probabilities.

The ski club is holding a raffle to raise money. They sold 100 tickets. All of the tickets are placed in a jar. One ticket will be pulled out of the jar at random, and the winner will receive a prize. Cherie bought one raffle ticket.

Find the probability she will win the prize.

Convert the fraction to a decimal.

Solution

Solution

This table outlines the step-by-step process to calculate the probability of Cherie winning a prize, applying the definition of probability.
What are you asked to find? The probability Cherie wins the prize.
What is the number of favorable outcomes? 1, because Cherie has 1 ticket.
Use the definition of probability. Probability of an event=number of favorable outcomestotal number of outcomes
Substitute into the numerator and denominator. Probability Cherie wins=1100
Illustrates converting a probability from fractional form (1/100) to its decimal equivalent (0.01).
Convert the fraction to a decimal.
Write the probability as a fraction. Probability=1100
Convert the fraction to a decimal. Probability=0.01

Three women and five men interviewed for a job. One of the candidates will be offered the job.

Find the probability the job is offered to a woman.

Convert the fraction to a decimal.

Solution

Solution

This table outlines the steps involved in calculating the probability of a specific event, using the example of a woman being offered a job.
What are you asked to find? The probability the job is offered to a woman.
What is the number of favorable outcomes? 3, because there are three women.
What are the total number of outcomes? 8, because 8 people interviewed.
Use the definition of probability. Probability of an event=number of favorable outcomestotal number of outcomes
Substitute into the numerator and denominator. Probability=38
This table illustrates the step-by-step conversion of a probability expressed as a fraction to its decimal form.
Convert the fraction to a decimal.
Write the probability as a fraction. Probability=38
Convert the fraction to a decimal. Probability=0.375

Key Concepts

  • Calculate the mean of a set of numbers.
    1. Write the formula for the mean mean=sum of values in data setn
    2. Find the sum of all the values in the set. Write the sum in the numerator.
    3. Count the number, n, of values in the set. Write this number in the denominator.
    4. Simplify the fraction.
    5. Check to see that the mean is reasonable. It should be greater than the least number and less than the greatest number in the set.
  • Find the median of a set of numbers.
    1. List the numbers from least to greatest.
    2. Count how many numbers are in the set. Call this n.
    3. Is n odd or even?
      If n is an odd number, the median is the middle value.
      If n is an even number, the median is the mean of the two middle values
  • Identify the mode of a set of numbers.
    1. List the data values in numerical order.
    2. Count the number of times each value appears.
    3. The mode is the value with the highest frequency.

Practice Makes Perfect

Calculate the Mean of a Set of Numbers

In the following exercises, find the mean.

3, 8, 2, 2, 5

Solution

4

6, 1, 9, 3, 4, 7

65, 13, 48, 32, 19, 33

Solution

35

34, 45, 29, 61, and 41

202, 241, 265, 274

Solution

245.5

525, 532, 558, 574

12.45, 12.99, 10.50, 11.25, 9.99, 12.72

Solution

11.65

28.8, 32.9, 32.5, 27.9, 30.4, 32.5, 31.6, 32.7

Four girls leaving a mall were asked how much money they had just spent. The amounts were $0, $14.95, $35.25, and $25.16. Find the mean amount of money spent.

Solution

$18.84

Juan bought 5 shirts to wear to his new job. The costs of the shirts were $32.95, $38.50, $30.00, $17.45, and $24.25. Find the mean cost.

The number of minutes it took Jim to ride his bike to school for each of the past six days was 21, 18, 16, 19, 24, and 19. Find the mean number of minutes.

Solution

19.5 minutes

Norris bought six books for his classes this semester. The costs of the books were $74.28, $120.95, $52.40, $10.59, $35.89, and $59.24. Find the mean cost.

The top eight hitters in a softball league have batting averages of .373, .360, .321, .321, .320, .312, .311, and .311. Find the mean of the batting averages. Round your answer to the nearest thousandth.

Solution

0.329

The monthly snowfall at a ski resort over a six-month period was 60.3, 79.7, 50.9, 28.0, 47.4, and 46.1 inches. Find the mean snowfall.

Find the Median of a Set of Numbers

In the following exercises, find the median.

24, 19, 18, 29, 21

Solution

21

48, 51, 46, 42, 50

65, 56, 35, 34, 44, 39, 55, 52, 45

Solution

45

121, 115, 135, 109, 136, 147, 127, 119, 110

4, 8, 1, 5, 14, 3, 1, 12

Solution

4.5

3, 9, 2, 6, 20, 3, 3, 10

99.2, 101.9, 98.6, 99.5, 100.8, 99.8

Solution

99.65

28.8, 32.9, 32.5, 27.9, 30.4, 32.5, 31.6, 32.7

Last week Ray recorded how much he spent for lunch each workday. He spent $6.50, $7.25, $4.90, $5.30, and $12.00. Find the median.

Solution

$6.50

Michaela is in charge of 6 two-year olds at a daycare center. Their ages, in months, are 25, 24, 28, 32, 29, and 31. Find the median age.

Brian is teaching a swim class for 6 three-year olds. Their ages, in months, are 38,41,45,36,40,and42. Find the median age.

Solution

40.5 months

Sal recorded the amount he spent for gas each week for the past 8 weeks. The amounts were $38.65, $32.18, $40.23, $51.50, $43.68, $30.96, $41.37, and $44.72. Find the median amount.

Identify the Mode of a Set of Numbers

In the following exercises, identify the mode.

2, 5, 1, 5, 2, 1, 2, 3, 2, 3, 1

Solution

2

8, 5, 1 , 3, 7, 1 , 1, 7 , 1, 8 , 7

18, 22, 17, 20, 19, 20, 22, 19, 29, 18, 23, 25, 22, 24, 23, 22, 18, 20, 22, 20

Solution

22

42, 28, 32, 35, 24, 32, 48, 32, 32, 24, 35, 28, 30, 35, 45, 32, 28, 32, 42, 42, 30

The number of children per house on one block: 1, 4, 2, 3, 3, 2, 6, 2, 4 , 2, 0, 3, 0.

Solution

2 children

The number of movies watched each month last year: 2, 0, 3, 0, 0, 8, 6, 5, 0, 1, 2, 3.

The number of units being taken by students in one class: 12, 5, 11, 10, 10, 11, 5, 11, 11, 11, 10, 12.

Solution

11 units

The number of hours of sleep per night for the past two weeks: 8, 5 , 7, 8, 8, 6 , 6, 6, 6, 9, 7, 8, 8, 8.

Use the Basic Definition of Probability

In the following exercises, express the probability as both a fraction and a decimal. (Round to three decimal places, if necessary.)

Josue is in a book club with 20 members. One member is chosen at random each month to select the next month’s book. Find the probability that Josue will be chosen next month.

Solution

120,0.05

Jessica is one of eight kindergarten teachers at Mandela Elementary School. One of the kindergarten teachers will be selected at random to attend a summer workshop. Find the probability that Jessica will be selected.

There are 24 people who work in Dane’s department. Next week, one person will be selected at random to bring in doughnuts. Find the probability that Dane will be selected. Round your answer to the nearest thousandth.

Solution

124,0.04160.042

Monica has two strawberry yogurts and six banana yogurts in her refrigerator. She will choose one yogurt at random to take to work. Find the probability Monica will choose a strawberry yogurt.

Michel has four rock CDs and six country CDs in his car. He will pick one CD to play on his way to work. Find the probability Michel will pick a rock CD.

Solution

25,0.4

Noah is planning his summer camping trip. He can’t decide among six campgrounds at the beach and twelve campgrounds in the mountains, so he will choose one campground at random. Find the probability that Noah will choose a campground at the beach.

Donovan is considering transferring to a 4-year college. He is considering 10 out-of state colleges and 4 colleges in his state. He will choose one college at random to visit during spring break. Find the probability that Donovan will choose an out-of-state college.

Solution

57,0.714285———0.714

There are 258,890,850 number combinations possible in the Mega Millions lottery. One winning jackpot ticket will be chosen at random. Brent chooses his favorite number combination and buys one ticket. Find the probability Brent will win the jackpot. Round the decimal to the first digit that is not zero, then write the name of the decimal.

Everyday Math

Joaquin gets paid every Friday. His paychecks for the past 8 Fridays were $315, $236.25, $236.25, $236.25,$315, $315, $236.25, $393.75. Find the mean, median, and mode.

Solution
  1. $285.47
  2. $275.63
  3. $236.25

The cash register receipts each day last week at a coffee shop were $1,845, $1,520, $1,438, $1,682, $1,850, $2,721, $2,539. Find the mean, median, and mode.

Writing Exercises

Explain in your own words the difference between the mean, median, and mode of a set of numbers.

Solution

Answers will vary.

Make an example of probability that relates to your life. Write your answer as a fraction and explain what the numerator and denominator represent.

Self Check

After completing the exercises, use this checklist to evaluate your mastery of the objectives of this section.

A self-assessment table for statistical skills, including calculating mean, median, mode, and using probability, with options to mark confidence levels: Confidently, With some help, or No-I don't get it!.

After looking at the checklist, do you think you are well prepared for the next section? Why or why not?

mean
The mean of a set of n numbers is the arithmetic average of the numbers. The formula is mean=sum of values in data setn
median
The median of a set of data values is the middle value.
  • Half the data values are less than or equal to the median.
  • Half the data values are greater than or equal to the median.
mode
The mode of a set of numbers is the number with the highest frequency.