You’ve used probability to describe the likelihood of events occurring. In this lesson, you will investigate how you can calculate the probability of two or more events.

Probability is used to describe how likely something is to happen. For example, you may have heard the weather forecaster describe the chances that it will rain today. She is using probability to make that forecast.

Probability is formally defined as the ratio of the number of desired outcomes (what you want to happen) to the number of total possible outcomes (what could possibly happen). Probability is a ratio, so it can be expressed as a fraction, a decimal, or a percent.

The probability of rain today is 80%. That means that there is an 80% chance that the event of rain will occur.

Out of a dozen dyed eggs, there are 2 blue ones. The probability of randomly choosing a blue egg is $\frac{2}{12}$. Since this is only one event occurring, this is called a simple event.

There is a 7 out of 10 chance that someone will catch the flu if they don't get a flu shot. The probability of getting the flu is 0.7.