Multi-event Probability: Addition Rule
The Addition Rule is used to calculate the probability that either (or both) of 2 events will happen:
Addition Rule Formula
When calculating the probability of either one of two events from occurring, it is as simple as adding the probability of each event and then subtracting the probability of both of the events occurring:
We must subtract P(A and B) to avoid double counting!
Special Case: Mutually Exclusive Events
If the 2 events are mutually exclusive (or sometimes called disjoint), that is if they can't BOTH happen!
If they both cannot happen, P(A and B) is 0. Therefore,
Simplified formula when P(A and B) = 0, only useable when A and B are mutually exclusive events!
Intersection and Unions Visualized
We can illustrate the idea of mutually exclusive and non-mutually exclusive events using Venn Diagrams.
Example: Drawing a Queen of Hearts
Let's look at an example of how to use the Addition Rule to answer a question regarding a deck of 52 cards.
Q: What's the probability of drawing either a queen or a heart from deck of cards?
In this case, you can get a card that’s both a queen and a heart (the queen of hearts!) so we need to subtract that probability to avoid double counting. There are 4 queens, 13 hearts, and one queen of hearts:
P(Q or ❤️) = P(Q) + P(❤️) - P(Q and ❤️)
P(Q or ❤️) = 4/52 + 13/52 - 1/52
P(Q or ❤️) = 16/52 = 30.77%
Example Walk-Throughs with Worksheets
Video 1: Addition Rule Examples
Practice QuestionsQ1: Say we are rolling 2 die. What is the probability that either die lands on a 2?
Q2: What is the sample space of flipping 5 coins?