Mathematics-Monster.com
(#mm)

menu

Finding a Probability
(KS3, Year 7)

homeprobabilityfinding a probability
A probability is a measure of how likely (how probable) an event is to happen. Imagine you tossed a coin. What is the probability of the coin landing Heads up?

coin toss

How to Find a Probability

Question

What is the probability of getting Heads in a coin toss?
heads or tails

Step-by-Step:

1

Find the number of ways getting Heads can happen. find_probability_step_1 There is only 1 way of getting Heads.

2

Find the total number of outcomes. find_probability_step_2 There are 2 outomes: getting Heads and getting Tails.

3

Divide the number of ways of getting Heads (1) (found in Step 1) by the total number of outcomes (2) (found in Step 2).
Probability of getting heads = 1 ÷ 2 = 0.5

Answer:

The probability of getting Heads is 0.5. Using notation, if H is the event of a Heads coming up, the probability of the event is P(H). find_probability_answer We can also express this as a fraction (½) or a percentage (50%).

A Formula to Find a Probability

The formula for finding a probability is shown below:
$$Probability = \frac{Number~of~ways~an~event~can~happen}{Total~number~of~outcomes}$$
Let's use the formula in an example.

Question

What is the probability of rolling a 6 on a die?
dice sides

Step-by-Step:

1

Start with the formula.
$$Probability = \frac{Number~of~ways~an~event~can~happen}{Total~number~of~outcomes}$$

2

Find the number of ways the event can happen. Find the number of ways getting a 1 can happen.

find probability 2 step 2 There is only 1 way of rolling a 1.

3

Find the total number of outcomes. find_probability_2_step_3 There are 6 outomes: getting 1, 2, 3, 4, 5 and 6.

4

Substitute the number of ways the event can happen and the total number of outcomes into the formula.
$$Probability = \frac{1}{6}$$

Answer:

The probability of rolling a 1 is 16. Using notation, if 1 is the event of rolling a 1, the probability of the event is P(1). find_probability_2_answer We can also express this as a number (0.167) or a percentage (16.7%).

Lesson Slides

The slider below another real example of how to find a probability.

A Note on Notation

The probability of an event can be written as:
P(Event)
A letter or symbol can be used to represent an event. For example, let H be the event that a coin lands on Heads when it has been tossed. We can denote the probability of getting heads as:
P(H)
author logo

This page was written by Stephen Clarke.

You might also like...

Help Us Improve Mathematics Monster

  • Do you disagree with something on this page?
  • Did you spot a typo?
Please tell us using this form.

Find Us Quicker!

  • When using a search engine (e.g., Google, Bing), you will find Mathematics Monster quicker if you add #mm to your search term.

Share This Page

share icon

If you like Grammar Monster (or this page in particular), please link to it or share it with others.

If you do, please tell us. It helps us a lot!

Create a QR Code

create QR code

Use our handy widget to create a QR code for this page...or any page.