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USING BOTH ADDITION AND MULTIPLICATION
Feb 18’09
RULES OF PROBABILITY
Many probability problems can be represented by
tree diagrams
. When this is possible, the
addition and multiplication rules can be applied quite easily. To illustrate the use of tree
diagrams, let us consider the following problem.
Two balls are drawn at random
without replacement
from a box containing one each of
red,
blue
, and
white
balls. Find the probability of drawing
(in any order) one red and one blue ball.
The following outcomes (events) are possible:
1
st
drawing:
Event
R
1
:
red ball is selected
first
.
Event
B
1
:
blue ball is selected
first
.
Event
W
1
: white ball is selected
first
.
2
nd
drawing:
Event
R
2
:
red ball is selected
second
.
Event
B
2
:
blue ball is selected
second
.
Event
W
2
: white ball is selected
second
.
The tree diagram representing this experiment (see Fig. 2) shows
mutually exclusive
outcomes of
a first drawing and then outcomes of a second drawing. (M.V.: Each outcome of the whole
experiment is characterized not only by the
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This note was uploaded on 11/24/2011 for the course MATH 3000 taught by Professor Kzaer during the Spring '05 term at St. Johns College MD.
 Spring '05
 Kzaer
 Addition, Multiplication, Probability

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