MathBench > Probability

Laws of AND and OR

Some Fine Print Relating to the Law of OR


Once again, The Law of OR says, if you need to know the probability that one thing OR another will take place, just add their separate probabilities.

Of course its not (quite) that simple. Here's the fine print: the events in question have to be mutually exclusive. That means they cannot happen together. In Life's a Game, you will win one prize, but not two, or three, or four.

Here are some mutually exclusive events:

Here are some non-mutually exclusive events:

 

rain and snowFor each of these, does it describe mutually exclusive choices?

The chance of a shower or a storm in the forecast tomorrow?

 

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I think I have the answer: NO


keychainYou think you left your keys in your pocket or your backpack?

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I think I have the answer: YES


curly or not?

The chance that a curly-haired child has a father OR a mother with curly hair?

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I think I have the answer: NO