1. Theoretical Probability
10-6
Theoretical Probability
Warm Up
Lesson Presentation
Lesson Quiz
Holt Algebra 1

2. Warm Up
An experiment consists of spinning a spinner 8 times. The spinner lands on red 4 times, yellow 3 times, and green once. Find the experimental probability of each event.
1. The spinner lands on red.
2. The spinner does not land on green.
3. The spinner lands on yellow.

3. Objectives Determine the theoretical probability of an event.
Convert between probabilities and odds.

4. When the outcomes in the sample space of an experiment have the same chance of occurring, the outcomes are said to be equally likely.

5. The theoretical probability of an event is the ratio of the number of ways the event can occur to the total number of equally likely outcomes.

6. An experiment in which all outcomes are equally likely is said to be fair. You can usually assume that experiments involving coins and number cubes are fair.

7. Example 1A: Finding Theoretical Probability
An experiment consists of rolling a number cube. Find the theoretical probability of each outcome.
rolling a 5
There is one 5
on a number cube.

8. Example 1B: Finding Theoretical Probability
An experiment consists of rolling a number cube. Find the theoretical probability of each outcome.
rolling an odd number
There 3 odd
numbers on a cube.
= 0.5 = 50%

9. Example 1C: Finding Theoretical Probability
An experiment consists of rolling a number cube. Find the theoretical probability of each outcome.
rolling a number less than 3
There are 2
numbers less three.

10. The probability of an event can be written as P(event)
The probability of an event can be written as P(event). P(heads) means “the probability that heads will be the outcome.”
Reading Math

11. When you toss a coin, there are two possible outcomes, heads or tails
When you toss a coin, there are two possible outcomes, heads or tails. The table below shows the theoretical probabilities and experimental results of tossing a coin 10 times.

12. The sum of the probability of heads and the probability of tails is 1, or 100%. This is because it is certain that one of the two outcomes will always occur.
P(event happening) + P(event not happening) = 1

13. The complement of an event is all the outcomes in the sample space that are not included in the event. The sum of the probabilities of an event and its complement is 1, or 100%, because the event will either happen or not happen.
P(event) + P(complement of event) = 1

14. Example 2: Finding Probability by Using the Complement
A box contains only red, black, and white blocks. The probability of choosing a red block is , the probability of choosing a black block is . What is the probability of choosing a white block?
P(red) + P(black) + P(white) = 100%
Either it will be a white block or not.
25% + 50% + P(white) = 100%
75% + P(white) = 100%
–75% –75%
Subtract 75% from both sides.
P(white) = 25%

15. Odds are another way to express the likelihood of an event
Odds are another way to express the likelihood of an event. The odds in favor of an event describe the likelihood that the event will occur. The odds against an event describe the likelihood that the event will not occur.
Odds are usually written with a colon in the form a:b, but can also be written as a to b or .

17. The two numbers given as the odds will add up to the total number of possible outcomes. You can use this relationship to convert between odds and probabilities.

18. You may see an outcome called “favorable
You may see an outcome called “favorable.” This does not mean that the outcome is good or bad. A favorable outcome is the outcome you are looking for in a probability experiment.
Reading Math

19. Example 3A: Converting Between Odds and Probabilities
The probability of rolling a 2 on a number cube is . What are the odds of rolling a 2 ?
The probability of rolling a 2 is . There are 5 unfavorable outcomes and 1 favorable outcome, thus the odds are 1:5.
Odds in favor are 1:5.

20. Example 3B: Converting Between Odds and Probabilities
The odds in favor of winning a contest are 1:9. What is the probability of winning the contest?
The odds in favor of winning are 1:9, so the odds against are 9:1. This means there is 1 favorable outcome and 9 unfavorable outcomes for a total of 10 possible outcomes.
The probability of winning the contest is

21. Example 3C: Converting Between Odds and Probabilities
The odds against a spinner landing on red are 2:3. What is the probability of the spinner landing on red?
The odds against landing on red are 2:3, so the odds in favor are 3:2. This means there are 3 favorable outcomes and 2 unfavorable outcomes for a total of 5 possible outcomes.
The probability of landing on red is