Concepts

Probability is usually expressed as a fraction or decimal. Since the number of ways,a certain outcome may occur is always smaller or equal to the total number of outcomes, the probability of an event is a number from 0 through 1.

Rules

To calculate probability, write a fraction. The top number is the number of ways the desired outcome can occur. The bottom number is the total number of possible outcomes: Desired outcomeTotal possible outcomes. The fractioncan be converted to a decimal or a percent.

A probability of 1 means that the event will absolutely occur. 

A probability of 0 means that there is no chance the event will occur. 

Example

What is the probability of randomly picking a yellow or green cube?

Write your answer as a fraction.

Solution

1. The top number is the number of ways the desired outcome can occur. There are 3 yellow blocks and 1 green block, so there are 4 possibilities that one of the desired color will be chosen. 

2. The bottom number is the total number of possible outcomes. There are a total of 6 blocks. This number goes on the bottom of the fraction.

Prerequisite Skills

Understanding Probability (4.4.7)