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. In other words, 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 snowWhich of the following sets of choices are mutually exclusive?

the chance of rain or snow in the forecast tomorrow?

 

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I think I have the answer: NO


keychainthe likelihood that 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 your father OR your mother have curly hair?

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I think I have the answer: NO