Probability
UK Secondary (7-11)
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Outcomes of experiments
Lesson

We've already looked at how probabilities are used to describe the chance of an event happening. Words such as "impossible," "certain," or "even chance" describe the likelihood of an event.

Now let's look at how we can use maths and numbers to describe probabilities as well.

 

Flipping a Coin

 

Let's say we flipped a coin. What outcomes are possible? In other words, what can the coin land on?

 

 

It can either land on heads or tails. In other words, there is an even chance of the coin landing on heads or tails because the chance of it landing on heads is the same as landing on tails.

So what is the probability that when we flip the coin, we get a head?

Well there is a $1$1 out of $2$2 chance of that happening, so we can write the probability of getting a head:

  • as a fraction: $\frac{1}{2}$12
  • as a decimal: $0.5$0.5
  • or as a percentage: $50%$50%

Probability can be expressed as a fraction, decimal or percentage, so make sure you're comfortable converting between these forms.

 

Spin the Spinner

Let's say we spun this spinner. What outcomes are possible? Are the chances of landing on each colour equal?

Well we could land on green, blue, red and yellow but the chances of doing so aren't equal.

Careful!

Just because there a $4$4 colours doesn't mean the chances of landing on each one are equal ($\frac{1}{4}$14). The green section is bigger, so the probability of landing on it is greater. Similarly, the red section is smaller, so the probability of landing on it is smaller. 

So what is the probability of landing on red?

Well, the picture shows $8$8 equal pieces. $1$1 of those is coloured red, so the probability of landing on red is $\frac{1}{8}$18

Now it's your turn!

Practice writing probabilities as fractions with this applet.

Discuss your answers and how you would express these probabilities in words (Certain, likely, even chance, unlikely or impossible).

Examples

Question 1

Homer has a bag of marbles with $10$10 marbles. $8$8 of those marbles are red.

Express the chance of Homer picking a red marble as a decimal.

Think: There is an $8$8 out of $10$10 chance Homer will pick a red marble.

Do: $8$8 out of $10$10 is the same as $\frac{8}{10}$810 or "$8$8 tenths." As a decimal, we would write this as $0.8$0.8

 

Question 2

A coin was tossed $50$50 times. It landed on tails $5$5 times.

Write the probability of getting as tail as a percentage.

Think: How do we write $5$5 out of $50$50 as a percentage?

Do: "Percentage" means "out of $100$100," so let's work out this probability out of $100$100.

$5$5 out of $50$50 $=$= $\frac{5}{50}$550 (Multiply the fraction by $2$2 to change the denominator to $100$100)
  $=$= $\frac{10}{100}$10100 (Now convert to a percentage)
  $=$= $10%$10%  

Worked examples

Question 1

This spinner is spun.

  1. What is the chance it lands on purple?

  2. What is the chance it lands on blue?

  3. What is the chance it lands on yellow?

Question 2

A dice is rolled $100$100 times. The results are shown in the table.

Result Frequency
$1$1 $19$19
$2$2 $15$15
$3$3 $20$20
$4$4 $12$12
$5$5 $20$20
$6$6 $14$14
  1. Express the chance it lands on a three as a percentage.

Question 3

Yuri has a bag of marbles with $10$10 marbles. $1$1 of those marbles are blue.

  1. Express the chance of Yuri picking a blue marble as a percentage.

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