Section 3.3
Addition Rule
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Section 3.3 Objectives
• Determine if two events are mutually exclusive
• Use the Addition Rule to find the probability of two
events
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Mutually Exclusive Events
Mutually exclusive
• Two events A and B cannot occur at the same time
A
B
A and B are mutually
exclusive
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A
B
A and B are not mutually
exclusive
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Example: Mutually Exclusive Events
Decide if the events are mutually exclusive.
Event A: Roll a 3 on a die.
Event B: Roll a 4 on a die.
Solution:
Mutually exclusive (The first event has one outcome, a
3. The second event also has one outcome, a 4. These
outcomes cannot occur at the same time.)
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Example: Mutually Exclusive Events
Decide if the events are mutually exclusive.
Event A: Randomly select a male student.
Event B: Randomly select a nursing major.
Solution:
Not mutually exclusive (The student can be a male
nursing major.)
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The Addition Rule
Addition rule for the probability of A or B
• The probability that events A or B will occur is
 P(A or B) = P(A) + P(B) – P(A and B)
• For mutually exclusive events A and B, the rule can
be simplified to
 P(A or B) = P(A) + P(B)
 Can be extended to any number of mutually
exclusive events
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Example: Using the Addition Rule
You select a card from a standard deck. Find the
probability that the card is a 4 or an ace.
Solution:
The events are mutually exclusive (if the card is a 4, it
cannot be an ace)
Deck of 52 Cards
P (4 or ace)  P (4)  P (ace)
4
4


52 52
8

 0.154
52
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4♣
4♥
4♠
4♦
A♣
A♠ A♥
A♦
44 other cards
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Example: Using the Addition Rule
You roll a die. Find the probability of rolling a number
less than 3 or rolling an odd number.
Solution:
The events are not mutually exclusive (1 is an
outcome of both events)
Roll a Die
4
Odd
3
5
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6
Less than
1 three
2
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Solution: Using the Addition Rule
Roll a Die
4
Odd
3
6
Less than
1 three
5
2
P(less than 3 or odd )
 P(less than 3)  P(odd )  P(less than 3 and odd )
2 3 1 4
     0.667
6 6 6 6
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Example: Using the Addition Rule
The frequency distribution shows
the volume of sales (in dollars)
and the number of months in
which a sales representative
reached each sales level during
the past three years. If this sales
pattern continues, what is the
probability that the sales
representative will sell between
$75,000 and $124,999 next
month?
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Sales volume ($)
Months
0–24,999
3
25,000–49,999
5
50,000–74,999
6
75,000–99,999
7
100,000–124,999
9
125,000–149,999
2
150,000–174,999
3
175,000–199,999
1
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Solution: Using the Addition Rule
• A = monthly sales between
$75,000 and $99,999
• B = monthly sales between
$100,000 and $124,999
• A and B are mutually exclusive
P ( A or B )  P ( A)  P ( B )
7
9


36 36
16

 0.444
36
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Sales volume ($)
Months
0–24,999
3
25,000–49,999
5
50,000–74,999
6
75,000–99,999
7
100,000–124,999
9
125,000–149,999
2
150,000–174,999
3
175,000–199,999
1
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Example: Using the Addition Rule
A blood bank catalogs the types of blood, including
positive or negative Rh-factor, given by donors during
the last five days. A donor is selected at random. Find
the probability that the donor has type O or type A
blood.
Type O
Type A
Type B
Type AB
Total
Rh-Positive
156
139
37
12
344
Rh-Negative
28
25
8
4
65
184
164
45
16
409
Total
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Solution: Using the Addition Rule
The events are mutually exclusive (a donor cannot have
type O blood and type A blood)
Type O
Type A
Type B
Type AB
Total
Rh-Positive
156
139
37
12
344
Rh-Negative
28
25
8
4
65
184
164
45
16
409
Total
P(type O or type A)  P(type O )  P(type A)
184 164


409 409
348

 0.851
409
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Example: Using the Addition Rule
Find the probability that the donor has type B blood or
is Rh-negative.
Type O
Type A
Type B
Type AB
Total
Rh-Positive
156
139
37
12
344
Rh-Negative
28
25
8
4
65
184
164
45
16
409
Total
Solution:
The events are not mutually exclusive (a donor can have
type B blood and be Rh-negative)
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Solution: Using the Addition Rule
Type O
Type A
Type B
Type AB
Total
Rh-Positive
156
139
37
12
344
Rh-Negative
28
25
8
4
65
184
164
45
16
409
Total
P(type B or Rh  neg)
 P(type B)  P(Rh  neg)  P(type B and Rh  neg)
45
65
8
102




 0.249
409 409 409 409
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Section 3.3 Summary
• Determined if two events are mutually exclusive
• Used the Addition Rule to find the probability of two
events
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