Probability runs from 0, meaning impossible, to 1, meaning certain.
Step 1: Let's Learn
Read it, or press Listen and follow the words.
For equally likely outcomes
Count the outcomes you want, divide by the total number of outcomes.
An example
A fair die has 6 faces and 3 of them are even, so the probability of an even number is 3/6, or 1/2.
The rest of the time
The probability something does not happen is 1 minus the probability it does.
A number from 0 to 1
Probability measures how likely something is, on a scale from 0 for impossible to 1 for certain. A half means equally likely either way. Every probability sits in that range — a value outside it signals an error.
Calculating a simple probability
When all outcomes are equally likely, the probability is the number of favourable outcomes divided by the total. Rolling a 4 on a fair die is 1/6. The equally-likely assumption is doing real work and is worth checking.
The probability of not happening
All the possibilities together have probability 1, so the chance of something not happening is 1 minus the chance that it does. If rain is 0.3 likely, no rain is 0.7. Sometimes the complement is much easier to compute.
Fractions, decimals and percentages
1/4, 0.25 and 25% are the same probability. Percentages are common in weather and news, fractions in dice problems, decimals in calculations. Being fluent in all three is what lets you compare claims from different sources.
Step 2: Try It Yourself
Tap and try it out.
3/6
1/2
3/6 is equal to 1/2.
Compare how much of the whole each one covers.
Step 3: Watch an Example
One step at a time.
Watch Ana Work Out a Probability
A bag holds 4 red and 6 blue counters.
- Step 1
She counts the total: 4 + 6 = 10 counters.
Step 4: Your Turn
Practice makes it stick.
The Bag
Problem 1 of 2
3 red and 7 blue. Out of 10, how many outcomes give red?
The Die
Problem 2 of 2
A fair die. Out of 6, how many outcomes are greater than 4?
Count the Outcomes
1 of 4
A fair die. Out of 6, how many outcomes are odd?
2 of 4
5 red and 5 blue. Out of 10, how many give blue?
3 of 4
What is the probability of something certain?
4 of 4
What is the probability of something impossible?
Step 5: Quick Check
Show what you know.
Question 1 of 2
A fair die. Out of 6, how many outcomes are less than 3?
Question 2 of 2
What range does a probability lie in?
What You Learned
- Probability runs from 0 to 1.
- For equally likely outcomes, divide wanted by total.
- The chance something does not happen is 1 minus the chance it does.