If two events are mutually exclusive, then they can't both be true/occur at the same time. For example, if I flip a coin, the answer can't be both heads and tails.

Another example is when getting your results for an exam. You are told that to pass you must obtain 40 marks minimum and you got 40. You have without a doubt passed, if you were to get 39 then you would have failed you can't fail and pass at the same time, either one or the other can occur.

With mutually exclusive events A and B, we use the rule:

P(A or B) = P(A) + P(B)

Which means the following:

The Probability of (A or B) is equal to the Probability of (A) + the Probability of (B)

Let's substitue the rule with an example of flipping a coin to illustrate how we would use this rule during our exam:

The Probability of (getting a Heads or Tails) is equal to the Probability of (getting a heads) + the Probability of (getting a tails)

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