Rule of Sum or Addition Principle
Definition: The Rule of Sum, also known as the Addition Principle or the Counting Principle, is a fundamental concept in combinatorics. It provides a systematic method for counting the number of possible outcomes when dealing with mutually exclusive events or choices.
Key Concepts:

Rule of Sum (Addition Principle): The Rule of Sum states that if there are "m" ways to do one thing and "n" ways to do another thing, but these events are mutually exclusive (meaning they cannot happen simultaneously), then there are "m + n" ways to do either one of these things. In other words, the total number of outcomes for a set of mutually exclusive events is the sum of the number of outcomes for each event.

Mutually Exclusive Events: The Rule of Sum is applied when you have a situation in which two or more events cannot occur at the same time. For example, when counting the number of ways to select a red or a green ball from a jar, these events are mutually exclusive because you cannot select both a red and a green ball simultaneously.

Example 1: Selecting a Ball from a Jar Let's consider an example where you want to count the number of ways to select either a red or a green ball from a jar containing red and green balls.
 There are 3 red balls in the jar.
 There are 2 green balls in the jar.
To find the total number of ways to select either a red or a green ball, apply the Rule of Sum: 3 (ways to select red) + 2 (ways to select green) = 5 ways to select a ball.

Example 2: Choosing a Dessert Suppose you want to count the number of possible dessert choices for a meal. You can either have ice cream or cake for dessert.
 There are 4 different flavors of ice cream to choose from.
 There are 3 types of cake available.
Using the Rule of Sum, you can calculate the total number of dessert choices: 4 (ice cream flavors) + 3 (cake types) = 7 dessert choices.
 Example 3: Suppose you have 3 hats, hats A, B, and C, and 2 coats, Coats 1 and 2, in your closet. Assuming that you feel comfortable with wearing either a hat or a coat. How many different choices of hat/coat combinations do you have? The number of different choices of hat/coat combinations is 3 + 2 = 5.
 Example 4: A restaurant offers 5 choices of appetizer and 4 choices of dessert. A customer can choose to eat either an appetizer or a dessert. Assuming that all food choices are available, how many different possible meals does the restaurant offer? The number of different possible meals the restaurant offers is 5 + 4 = 9.
 Example 5: In a game, a player can either roll a 6sided die or flip a coin. How many different possible outcomes are there? The number of different possible outcomes is 6 + 2 = 8.

Additional Notes
 The Rule of Sum can be extended to more than two events. For example, if there are $n$ mutually exclusive events, then the total number of possible outcomes is the sum of the number of possible outcomes of each event:
# of possible outcomes = ${n}_{1}$ + ${n}_{2}$ + ... + ${n}_{n}$
 The Rule of Sum can also be used to count the number of ways to arrange objects in a specific order. For example, if you have 3 different toys, there are $3!$ ways to arrange them in a row. However, if the order of the toys does not matter, then there are only 3 ways to arrange them.

NonMutually Exclusive Events: It's important to note that the Rule of Sum is not used when events are not mutually exclusive. If the same event could result in multiple outcomes simultaneously, you would use the Rule of Product (Multiplication Principle) instead.

Application in Probability: The Rule of Sum is also applied in probability theory to calculate the probabilities of mutually exclusive events occurring. It helps in finding the total probability of an outcome by adding the individual probabilities of each mutually exclusive event.