
Ms. Garcia Math 


Probability 
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is the chance that an event will happen. 


The easiest way to get the probability of a condition is to list all possible outcomes, and count the ones that fit the condition. 




Tree diagrams 
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A tree diagram represents a sequence of events, showing all possible outcomes 


From the tree diagram, we could count all of the possible outcomes. The set of possible outcomes is: HH, HT, TH and TT # possible outcomes = 4 




Working out probabilies by counting 
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Once you have listed all possible outcomes, then you can work out the probabilities quite easily, counting the ones that fit the condition. 
Example: What is the probability of flipping a coin 2 times and it coming up "heads" both times? 

Answer: # of favorable outcomes = 1, # of possible outcomes = 4, P(HH) = 1/4 




Counting Principle 
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if one event has m possible outcomes and a second independent event has n possible outcomes, then there are m x n total possible outcomes for the two events together. 





Flip a coin and roll a die 
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There are 2 ways that you can flip a coin and 6 ways that you can roll a die. There are then 2x6=12 ways that you can flip a coin and roll a die. 







