# 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:

• Your friend is pregnant OR not pregnant;
as we all know, there's no such thing as a little pregnant.
• Your mother offers you cake OR a brownie for dessert.
You can't get both!
• You have one dollar. You can buy fries OR a donut OR a small milkshake,
but after you buy one thing, your money's gone, you're out of luck.

Here are some non-mutually exclusive events:

• Your grandmother offers you cake OR a brownie OR both.
But take both dear, you're too skinny!
• Your friend has a cold OR a flu OR maybe both,
she looks so miserable.
• You just got paid, to celebrate you could get fries OR a donut OR a small milkshake
OR any combination of the above.

# Which of the following sets of choices are mutually exclusive?

### the chance of rain or snow in the forecast tomorrow?

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• I need a hint ... : it could be both

• ...another hint ... : not mutually exclusive

#### I think I have the answer: NO

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• I need a hint ... : keys can only be in a single place at once
• ...another hint ... : the two possibilities are mutually exclusive (but again, not exhaustive)

### the chance that your father OR your mother have curly hair?

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• I need a hint ... : its possible that both of your parents could have curly hair
• ...another hint ... : these are not mutually exclusive