Probability
UK Secondary (7-11)
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Mixed Probability and Sample Spaces
Lesson

When answering questions involving probability, it helps to have in front of us a way to represent all possible outcomes. 

We can do this by creating what's called a sample space.

A sample space can take on many forms and the most common include:

  • A list
  • An array
  • A table
  • A tree diagram
  • A Venn diagram
  • A two-way table

A List

Example 1:

Let's say we're interested in how many Personal Identification Numbers (PINs) can be made from the digits $1$1, $2$2, $3$3 and $8$8.

To see all options in front of us we can systematically list all combinations as follows:

$1238$1238 $2138$2138 $3128$3128 $8123$8123
$1283$1283 $2183$2183 $3182$3182 $8132$8132
$1823$1823 $2318$2318 $3218$3218 $8213$8213
$1832$1832 $2381$2381 $3281$3281 $8231$8231
$1328$1328 $2813$2813 $3812$3812 $8312$8312
$1382$1382 $2831$2831 $3821$3821 $8321$8321

At a glance we can now see there are $24$24 combinations and we can easily use our list to determine probabilities.

a)  What is the probability that a PIN starts with a $3$3 and ends with a $2$2?

Think: We look down the column where all PINs start with $3$3 and count those that also end with a $2$2.

Do: $\frac{2}{24}$224

b)  What is the probability that a PIN starts with a $3$3 or ends with a $2$2?

Think: This time we need to count all the PINs that either start with a $3$3, end with a $2$2 OR both.

Do: $\frac{10}{24}$1024

An Array

Example 2:

Suppose we're interested in the outcomes from rolling a normal six sided dice and spinning this spinner.

A great way to systematically do this is by creating what we can call an array (you can also simply call it a table).

  red yellow blue green
1 1,red 1,yellow 1,blue 1,green
2 2,red 2,yellow 2,blue 2,green
3 3,red 3,yellow 3,blue 3,green
4 4,red 4,yellow 4,blue 4,green
5 5,red 5,yellow 5,blue 5,green
6 6,red 6,yellow 6,blue 6,green

We can now easily see that there are 24 total possible outcomes.

A Table

A table is similar to an array, but instead of listing with the use of commas what results from the two categories in the experiment, we can use a table to represent what can happen when we perform a calculation using the two categories in an experiment.

Example 3:

A six sided dice is rolled and the following spinner is spun.

The sum of the results is then recorded.

a)  Use a table to represent all possible outcomes of this experiment.

  Spinner
  1 2 3 4 5
Dice 1 2 3 4 5 6
2 3 4 5 6 7
3 4 5 6 7 8
4 5 6 7 8 9
5 6 7 8 9 10
6 7 8 9 10 11

b)  Determine the probability that an even number was spun and a sum that is a multiple of four was obtained.

Think:  On the spinner there are two even numbers, $2$2 and $4$4. We now look down those columns for any sums that are a multiple of $4$4.

Do:  $\frac{3}{30}$330

c)  Determine the probability the sum was greater than $7$7 or a multiple of $4$4.

Think:  We need to look in the table for all sums greater than $7$7, or that are a multiple of $4$4, or both. I've highlighted those in the table below.

  Spinner
  1 2 3 4 5
Dice 1 2 3 4 5 6
2 3 4 5 6 7
3 4 5 6 7 8
4 5 6 7 8 9
5 6 7 8 9 10
6 7 8 9 10 11

Do:  $\frac{13}{30}$1330

A Tree Diagram

We've discussed tree diagrams in detail before, you can review that here if you'd like to.

A Venn Diagram and a Two-Way Table

Venn diagrams and Two-way tables are two different formats for representing the same ideas. You can review those here. 

Example 4:

Sam thought that most students at her school had blonde hair and blue eyes. She surveyed 30 students at her school and found that 20 had blonde hair and 17 had blue eyes. Seven students had neither blonde hair nor blue eyes.

a)  Draw a Venn diagram to represent this situation.

We can begin by putting the information we have into a Venn diagram. Notice how I've used the rectangle in the top right and the loops in the circles to keep track of the totals.

Now we need to complete all sections in the regions of the Venn diagram. We can do so with the following calculations.

b)  Create a two-way table to represent the same information.

  Blonde Not Blonde Total
Blue 14 3 17
Not Blue 6 7 13
Total 20 10 30

Now check that you agree with the following probabilities and are able to use each diagram.

c)  Calculate the probability that a student did not have blonde hair.

$\frac{10}{30}$1030

d)  Calculate the probability that a student did not have blonde hair or did not have blue eyes.

$\frac{7+3+6}{30}$7+3+630

=$\frac{16}{30}$1630

More Worked Examples

QUESTION 1

The following two spinners are spun and the result of each spin is recorded.

  1. Complete the following table to represent all possible combinations.

    Spinner $A$A $B$B $C$C
    $1$1 $1,A$1,A $\editable{},\editable{}$, $\editable{},\editable{}$,
    $2$2 $\editable{},\editable{}$, $\editable{},\editable{}$, $2,C$2,C
    $3$3 $\editable{},\editable{}$, $\editable{},\editable{}$, $\editable{},\editable{}$,
  2. State the total number of possible outcomes.

  3. What is the probability that the spinner lands on a consonant and an even number?

  4. What is the probability that the spinner lands on a vowel or a prime number?

QUESTION 2

$200$200 people were surveyed about the types of cuisine they’d eaten in the last month.

The results are shown in the Venn diagram below.

  1. Given that the number of people who ate both Japanese and French is $21$21, find the value of $w$w in the diagram.

  2. Given that the number of people who ate French is $86$86, find the value of $x$x in the diagram.

  3. Given that the number of people who ate Japanese is $58$58, find the value of $y$y in the diagram.

  4. Find the value of $z$z in the diagram.

  5. Calculate the probability that randomly selected person has eaten Japanese and French .

  6. Calculate the probability that a randomly selected person has not eaten Korean .

  7. Calculate the probability that a randomly selected person has eaten Korean or French , but not Japanese .

QUESTION 3

Luke can’t remember the order of his PIN for his EFTPOS card, but he knows it contains the digits in $2853$2853.

  1. The table below lists all the possible combinations of Luke’s PIN beginning with $2$2 and $8$8.

    $2853$2853 $2385$2385 $8523$8523
    $2835$2835 $2358$2358 $8532$8532
    $2583$2583 $8253$8253 $8325$8325
    $2538$2538 $8235$8235 $8352$8352

    Write all the OTHER possible combinations of pins. Write them on the same line, separated by commas.

  2. State the total number of possible outcomes.

  3. What is the probability his PIN starts with $5$5?

  4. What is the probability his PIN has $5$5 followed immediately by $2$2?

  5. What is the probability his PIN starts with $8$8, or ends with $3$3, or both?

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