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 $85, 88,$ and $94$. To find the mean score, he would add them and divide by $3$:

His mean test score is $89$ points.

Example. Find the mean of the numbers $8, 12, 15, 9,$ and $6$.

Write the formula for the mean, write the sum of the numbers in the numerator, and count how many numbers are in the set — there are $5$ numbers, so $n = 5$:

Check to see that the mean is “typical”: $10$ is neither less than $6$ nor greater than $15$. The mean is $10$.

Example. The ages of the members of a family who got together for a birthday celebration were $16, 26, 53, 56, 65, 70, 93,$ and $97$ years. Find the mean age.

Write the sum of the numbers in the numerator, and count how many numbers are in the set — there are $8$, so $n = 8$:

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.

Example. For the past four months, Daisy’s cell phone bills were $\text{\textdollar}42.75$, $\text{\textdollar}50.12$, $\text{\textdollar}41.54$, $\text{\textdollar}48.15$. Find the mean cost of Daisy’s cell phone bills.

Count how many numbers are in the set — there are $4$ — and write the sum of all the numbers in the numerator:

Does $\text{\textdollar}45.64$ seem “typical” of this set of numbers? Yes, it is neither less than $\text{\textdollar}41.54$ nor greater than $\text{\textdollar}50.12$. The mean cost of her cell phone bill was $\text{\textdollar}45.64$.

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: $59, 60, 65, 68, 70$ inches. Dora is in the middle of the group. Her height, $65$ inches, 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.

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:

There is no single middle value — the heights of the six girls can be divided into two equal parts, $59, 60, 62$ and $65, 68, 70$. 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 $62$ and $65$, $\tfrac{62+65}{2}$. The median height is $63.5$ inches.

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.

Example. Find the median of $12, 13, 19, 9, 11, 15,$ and $18$.

List the numbers in order from smallest to largest: $9, 11, 12, 13, 15, 18, 19$. Count how many numbers are in the set — $n = 7$, which is odd, so the median is the middle value. The middle is the number in the 4th position, so the median of the data is $13$.

Example. Kristen received the following scores on her weekly math quizzes: $83, 79, 85, 86, 92, 100, 76, 90, 88,$ and $64$. Find her median score.

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 — $n = 10$, which is even, so the median is the mean of the two middle values, the 5th and 6th numbers, $85$ and $86$:

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: $2, 15, 8, 3, 8, 5, 8$. If we list the numbers in order it is easier to identify the one with the highest frequency:

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

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

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

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.

Example. The data lists the heights, in inches, of $31$ students in a statistics class. Identify the mode.

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 $\tfrac{3}{5}$ 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:

Converting the fraction $\tfrac{3}{5}$ to a decimal, we would say there is a $0.6$ probability of choosing a banana. This basic definition of probability assumes that all the outcomes are equally likely to occur.

Example. 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. (a) Find the probability she will win the prize. (b) Convert the fraction to a decimal.

(a) There is $1$ favorable outcome (Cherie has $1$ ticket) out of $100$ total outcomes:

(b) Convert the fraction to a decimal: $\text{Probability} = 0.01$.

Example. Three women and five men interviewed for a job. One of the candidates will be offered the job. (a) Find the probability the job is offered to a woman. (b) Convert the fraction to a decimal.

(a) The number of favorable outcomes is $3$ (three women), and the total number of outcomes is $8$ (eight people interviewed):

(b) Convert the fraction to a decimal: $\text{Probability} = 0.375$.

Key terms

mean — the arithmetic average of a set of $n$ numbers, found by dividing the sum of the values by $n$. median — the middle value of a set of data (or the mean of the two middle values, when there is an even number of values). mode — the value in a set of numbers with the highest frequency. frequency — the number of times a value occurs in a data set. probability — the number of favorable outcomes divided by the total number of possible outcomes.

This section is adapted from Prealgebra 2e, Section 5.5: Averages and Probability by Lynn Marecek, MaryAnne Anthony-Smith, and Andrea Honeycutt Mathis, © OpenStax, licensed under CC BY-NC-SA 4.0. Access the original for free at openstax.org. Changes: recreated the calendar figure as a plain list of running distances and the frequency tables as markdown tables; omitted the Be Prepared quiz, Media links, and end-of-section exercises; and converted the practice problems (“Try Its”) into interactive exercises with instant feedback.