1. What is Probability?
The study of probability helps us figure out the likelihood of something happening.
In math we call this “something happening” or an “event”
Probability is a way of expressing what the chances are that an event will occur

2. The term probability is common
Weather forecast - 30 % chance of rain
Gambling - two dice will produce a sum of 7 is 1/6
Things to know:
1st thing:
What the probability statements mean
2nd thing:
Then we consider how we determine the numerical value for that probability

3. 1 2 3 4 5 6 Example: Number of ways 2 dice would produce a sum of 7
Die 1
Die 2 1 2 3 4 5 6

4. 36 different combinations
Shows all the different ways the dice can land
Shows all the ways you could roll a seven
6 of the 36 produce the sum of 7
Must assume that each of the 36 combinations are equally likely to occur
So there is a 1/6 chance that you the sum of the dice will be 7

5. Example: Jar with 4 red marbles and 6 blue marbles
Want to find the probability of drawing a red marble at random.
Favorable outcome = drawing a red marble

6. Ways to Express Probability
As a fraction~ 4/10 = 2/5
As a decimal ~ 4/10 = .4
As a percent ~ 4/10 = 40/100 = 40%
Unlikely events have a probability near zero
Likely events have probabilities near 1

7. What is the total number of possible outcomes?
Is called a sample space
Sample space is a set consisting of all the possible outcomes of an event.
The number of different ways you can choose something from the sample space is the total number of possible outcomes.

8. We only have 2 events with our red and blue marbles
Either we pick a red marble or a blue marble
If you don’t do the first, then you must do the second
So the probability of picking a red marble plus the probability of picking a blue marble equals 1 or 100%
Sum = 4/10 + 6/10 = 10/10 = 1

9. So we have 4 red marbles and 6 blue marbles in our jar
Sample space = all ten marbles
because we are likely to draw any
one of them
Favorable outcomes = # of red marbles = 4
Possible outcomes = total # of marbles = 10
4/10 reduces to 2/5
Probability of drawing a red marble where all outcomes are equally likely is 2/5
(sample space)

10. When one event occurs:
Probability of picking a red marble was 4/10 or 2/5
Sample space = 10 marbles in the jar
So the probability of not picking a red marble
1 = 10/10
10/10 -4/10 = 6/10 or 3/5
(this is also the probability of picking a blue marble)

11. When two events that are equally likely occur:
You draw 1 marble from the 10
Then I draw another marble from the nine that remain
What is the probability that I will draw a blue one first?
What is the probability that you will draw a red one second?

12. Your probability of drawing a blue one is 6/10
After you draw there are only 9 marbles left and 4 of those are red, so the probability that I will draw a red one is 4/9
When there are 2 events, the second outcome is dependent on the first.

13. When two or more events occur that are not all equally likely:
A. you draw a blue marble and then I draw a blue marble B. you draw a blue marble and then I draw a red marble C. you draw a red marble and then I draw a blue marble D. you draw a red marble and then I draw a red marble There are four possibilities but they are not all equally likely. Two separate events with the work “and”, there the outcome of the second is dependent on the outcome of the first we multiply.

14. Example A:
Your probability of drawing a blue marble (6/10 = 3/5) X my probability of drawing another blue marble which would be 5/9

15. Example B:
Your probability of drawing a blue marble (3/5) X my probability of drawing a red marble (4/9)

16. What can you learn from the chart?
Classroom Exercise
What can you learn from the chart?

17. Average Distribution of Colors
Plain
Peanut
Crispy
Dark Chocolate
Peanut Butter
Almond
brown 13 12 17 10
red 14 15 20
yellow
green 24 23
orange 16
blue