Probability of an event E + probability of the event not E is equal to ...............

Answer

To solve the question, "Probability of an event E + probability of the event not E is equal to ...............", we can follow these steps:

### Step-by-Step Solution:

1. **Understanding Events**:
- Let \( P(E) \) represent the probability of an event \( E \).
- The event "not E" is represented as \( E' \) or the complement of \( E \).

2. **Using the Complement Rule**:
- The probability of the event not happening (not E) is given by \( P(E') \).
- According to the complement rule in probability, we have:
\[ P(E') = 1 - P(E) \]

3. **Adding the Probabilities**:
- Now, we can add the probabilities of event \( E \) and its complement \( E' \):
\[ P(E) + P(E') = P(E) + (1 - P(E)) \]

4. **Simplifying the Expression**:
- Simplifying the above expression gives:
\[ P(E) + P(E') = P(E) + 1 - P(E) = 1 \]

5. **Conclusion**:
- Therefore, we conclude that:
\[ P(E) + P(E') = 1 \]
- This means that the probability of an event \( E \) plus the probability of the event not \( E \) is equal to 1.

### Final Answer:
The probability of an event \( E \) plus the probability of the event not \( E \) is equal to **1**.

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