​Probability: refers to the likelihood of an event or combination of events occurring.

​Probabilities can fall anywhere from 0 to 1.

0 means the event will never occur.

1 means the event will always occur.

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A probability between 0 and 1 means the event will sometimes occur; the higher the value, the greater the likelihood of the event.

​Probability Formulas

​Converting

​Example

​Independent Events

If the first event DOES NOT influence the second, we can use the following formula.

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Probability of Two Independent Events =

Probability of 1st Event × Probability of 2nd Event

​EXAMPLE (long way)

​Consider a game where you first flip a coin and then roll a die. How might we determine the probability of first flipping “heads” then rolling either a 5 or 6?

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​​Maybe create a tree diagram:

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​Number of Successful Outcomes =

Number of Possible Outcomes =

​EXAMPLE (short way)

​Consider a game where you first flip a coin and then roll a die. How might we determine the probability of first flipping “heads” then rolling either a 5 or 6?

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​​Probability of Two Independent Events =

Probability of 1st Event × Probability of 2nd Event

​Dependent Events

If the first event DOES influence the second, we can use the following formula.

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Probability of Two Dependent Events =

Probability of 1st Event × Probability of 2nd Event after the first event

​Example

There are eight shirts in your closet, four blue and four green. You randomly select one to wear on Monday and then a different one on Tuesday. What is the probability you wear blue shirts both days?

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Probability of Two Dependent Events =

Probability of 1st Event × Probability of 2nd Event after the first event