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# Probability

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```									Probability and Odds
PSSA Unit
   When the weather channel
predicts that the chance of
rain for the next day is 70%, it
is telling us that the likelihood
for this precipitation to occur is
7 chances out of 10.
What is Probability?
•   Probability is a numerical measure
indicating the chance or likelihood
for a particular event to occur.
* It is usually expressed as a ratio
namely by a fraction, decimal, or
percent.
Example:
   When we toss a coin in the air, it
is equally likely that it will land
heads up as tails up. Therefore,
the chance that this coin will land
heads up is one out of two, or 1/2.
   The set of all the possible outcomes
of a given experiment is called the
sample space.

   Probability of an event =
number of favorable events
total number of events
Dice
   A typical dice has 6 sides. Each side
contains a number from 1-6.
   If you roll a die, what is the probability of
getting:
1. A 4?
2. An odd number?
3. A number less than 3?
4. A number more than 6?
   We said that the probability of rolling
a 4 is 1/6.
   If we roll a die 30 times, how many
times am I likely to get a 4?

1
30 g  5 times
6
1. A 2, in 60 rolls.

2. An odd number, in 100 rolls.

3. A number less than 3, in 300 rolls.
Suppose the 8 letters of the words HOT
COCOA are written on cards, then
shuffled. If a card is chosen at random, the
probability that it will contain the letter 0 is
_____. What is the probability that it will
contain:

The letter C?
A vowel?
A letter in the word CHOCOLATE?
   Probability is often expressed
in terms of “odds”.

Odds in favor of an event 
number of favorable outcomes
number of unfavorable outcomes
Example
   There are 12 colored marbles in a
bag, and 3 of them are blue. What
are the odds for drawing a blue
marble from the bag?

number of favorable outcomes  3 1
 
number of unfavorable outcomes 9 3
Example
What are the odds that you will win the
prize if you hold 50 raffle tickets of
the 1500 tickets sold at a charity
affair?
number of favorable outcomes   50   1
    
number of unfavorable outcomes 1450 29
Spinners
   A spinner has a circle with twelve
numbers each numbered 1-12. What are
the odds that the pointer stops on the first
spin by a number divisible by 4?
   Numbers divisible by 4 include: 4,8, and
12.
3 1
 or 1 : 3
9 3

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