The Addition Rule: A Simple Guide to Probability

The Addition Rule is a helpful tool for figuring out probability. It's particularly useful for complicated questions in your Year 1 Math class. This rule helps us calculate the chance of either one of two events happening, especially when those events can’t happen at the same time.

What Do We Mean by Mutually Exclusive Events?

1. Mutually Exclusive Events:
   • These events cannot happen together. For example, when you roll a die, you can get a 1 or a 2, but you can’t get both on the same roll.

The Addition Rule Explained

2. The Addition Rule Formula:
   • This rule tells us that if we want to find out the chance of two mutually exclusive events, A and B, we can use this formula: [ P(A \cup B) = P(A) + P(B) ]
   • This just means you add the probabilities of each event together.

How to Solve Probability Problems

Here’s a simple way to solve tricky probability questions:

• Step 1: Identify the Events

  • Look closely at the problem and see what events you’re working with. Make sure to check if they are mutually exclusive.

• Step 2: Find Individual Probabilities

  • Determine the probability for each event. You can figure these out by doing experiments like flipping coins or rolling dice.

• Step 3: Use the Addition Rule

  • If the events can’t happen at the same time, just add their probabilities using the formula. For example, if ( P(A) = 0.2 ) and ( P(B) = 0.3 ): [ P(A \cup B) = 0.2 + 0.3 = 0.5 ]

• Step 4: Tackling Non-Mutually Exclusive Events

  • If the events can occur at the same time, you need to adjust the formula: [ P(A \cup B) = P(A) + P(B) - P(A \cap B) ]
  • Here, you subtract the probability of both events happening at once to avoid counting them twice.

By practicing these steps, you'll see how the Addition Rule can make understanding probability much easier!