Probability as Fractions
Probability measures how likely an event is to occur. We express probability as a fraction between 0 (impossible) and 1 (certain), where the numerator is the number of favourable outcomes.
What You Need to Know
Key Concept Diagram
Probability = number of favourable outcomes / total number of outcomes
Probability of 0 means impossible; probability of 1 means certain
All probabilities must be between 0 and 1 inclusive
Complementary events: P(event does not occur) = 1 - P(event occurs)
Key Vocabulary
Probability
A measure of how likely an event is, expressed as a fraction, decimal, or percentage
Favourable outcome
An outcome that satisfies the condition of the event being considered
Sample space
The set of all possible outcomes of an experiment
Complementary event
The event that the original event does NOT occur; probabilities add to 1
Knowledge Check
Select the correct answer for each question. Click "Check Answer" to see if you are right.
Question 1
A bag has 3 red and 5 blue marbles. What is the probability of picking a red marble?
Question 2
A fair die is rolled. What is the probability of getting an even number?
Question 3
The probability of rain tomorrow is 0.3. What is the probability that it does NOT rain?
Key Concepts Summary
- ●Probability = number of favourable outcomes / total number of outcomes
- ●Probability of 0 means impossible; probability of 1 means certain
- ●All probabilities must be between 0 and 1 inclusive
- ●Complementary events: P(event does not occur) = 1 - P(event occurs)