Complete the following question on page 179 (13a) for practice in finding the probability of mutually exclusive events.

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In both Lessons 3A and 3B, you have seen that the chance of an event not happening is referred to as a not probability . This occurs when you are dealing with complements.

P ( A ) + P (not A ) = 1

P ( A ) + P ( A' ) = 1

Examples 3 and 4 illustrate how complements can be used to solve a problem involving mutually exclusive events.

 

A fair 6-sided die is rolled. What is the probability that the number rolled is not a 5?

You want to find the probability of rolling not a 5 . This is mutually exclusive to rolling a 5 . These outcomes are also complementary.

The probability of not rolling a 5 on a fair die is or about 83%.