NOTES
Probability
Now that we know how to determine the number of outcomes, we are ready to find probability.
BIG IDEA #1: Probability is a number between ____ and ____ that describes how likely
something is to happen. A simple event is ______ thing happening, while a compound event
means _____ things happen at the same time.
The probability formula: (simple events)
P(event) =
Number of favorable outcomes
Number of possible outcomes
How to find the probability of a SIMPLE (one) event:
1. ____________ the number of possible outcomes – this is the denominator (bottom)
2. Then determine how many ways the _________ can occur – this is the numerator (top)
3. We will write probability three ways: as a ____________ fraction, a ____________, and
a _____.
Examples :
What is the probability of rolling a one on a die? P(1) =
What is the probability of rolling an even number on a die? P(even) =
What is the probability of rolling a three or a four on a die? P(3 or 4) =
The probability formula: (compound events)
P(A and B) = P(A) × P(B)
How to find the probability of COMPOUND (more than one) events:
1. Find the probability of each event _______________
2. ____________ the fractions from Step 1.
*Remember…to multiply fractions, do
_____ × ____, then
___________×___________.
Reduce!
Examples :
What is the probability of rolling a one on a die and then rolling a two on a die?
P(1 and 2) =
×
Practice:
=
Probability of
Probability of
rolling a one
rolling a two
1. What is the probability of rolling a seven on a die? P(7)
2. What is the probability of not rolling a five or a six? P(not 5 or 6)
3. What is the probability of rolling a number greater than 2? P(greater than 2)
4. What is the probability of rolling an even number on a die and then a three?
P(even and 3) =
×
=
Probability of
Probability of
rolling an even #
rolling a three
5. What is the probability of rolling a number greater than three and a number less than two?
P(greater than 3 and less than 2) =
×
=
6. What is the probability of picking a vowel from the word BANANA and picking a vowel from
the work SOUP?
P(vowel and vowel)
×
=
7. What is the probability of tossing a coin and landing on heads and rolling a die and getting
an odd number?