Pre-Algebra Chapter 6 - Ramsey School District

Reading Math
P(prime)
P(prime) is read as the
probability of rolling
a prime number.
Example 1
Find Probability
Suppose a number cube is rolled. What is
the probability of rolling a prime number?
There are 3 prime numbers on a number cube:
2, 3, and 5.
There are 6 possible outcomes: 1, 2, 3, 4, 5, and 6.
number of favorable outcomes
P(prime)
number of possible outcomes
3 6 or 1 2
So, the probability of rolling a prime number is 1 or 50%.
2
The set of all possible outcomes is called the sample space. For Example 1,
the sample space was {1, 2, 3, 4, 5, 6}. When you toss a coin, the sample space
is {heads, tails}.
Example 2
Find Probability
Suppose two number cubes are rolled. Find the probability of rolling an
even sum.
Make a table showing the sample space when rolling two number cubes.
1
2
3
4
5
6
1 2 3 4 5 6
(1, 1) (1, 2) (1, 3) (1, 4) (1, 5) (1, 6)
(2, 1) (2, 2) (2, 3) (2, 4) (2, 5) (2, 6)
(3, 1) (3, 2) (3, 3) (3, 4) (3, 5) (3, 6)
(4, 1) (4, 2) (4, 3) (4, 4) (4, 5) (4, 6)
(5, 1) (5, 2) (5, 3) (5, 4) (5, 5) (5, 6)
(6, 1) (6, 2) (6, 3) (6, 4) (6, 5) (6, 6)
There are 18 outcomes in which the sum is even.
So, P(even sum) 1 8
or 1 36
2 .
This means there is a 50% chance of rolling an even sum.
The probabilities in Examples 1 and 2 are called theoretical probabilities.
Theoretical probability is what should occur. Experimental probability is
what actually occurs when conducting a probability experiment.
Example 3
Find Experimental Probability
The table shows the results of an
Outcome Tally Frequency
experiment in which a coin was tossed.
Heads |||| |||| |||| 14
Find the experimental probability of
Tails |||| |||| | 11
tossing a coin and getting tails for this
experiment.
number of times tails occur 11
11
or
number of possible outcomes 14 11 2 5
11 The experimental probability of getting tails in this case is or 44%.
2 5
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Lesson 6-9 Probability and Predictions 311