Probability and counting rules

CHAPTER 4: PROBABILITY AND COUNTING RULES

4.1 Sample spaces and probability
Basic concepts

Processes such as flipping a coin, rolling die, or drawing a card from a deck are called probability
experiments.
A probability experiment is a chance process that leads to well-defined results called outcomes.
An outcome is the result of a single trial of a probability experiment.
A sample space is the set of all possible outcomes of a probability experiment.

Example 1:
Find the sample space for rolling two dice.

Solution
Since each die can land in six different ways, and two dice are rolled, the sample space can be presented
by a rectangular array. The sample space is the list of pairs of numbers in the chart.
Die 2
Die 1 1 2 3 4 5 6
1 (1,1) (1,2) (1,3) (1,4) (1,5) (1,6)
2 (2,1) (2,2) (2,3) (2,4) (2,5) (2,6)
3 (3,1) (3,2) (3,3) (3,4) (3,5) (3,6)
4 (4,1) (4,2) (4,3) (4,4) (4,5) (4,6)
5 (5,1) (5,2) (5,3) (5,4) (5,5) (5,6)
6 (6,1) (6,2) (6,3) (6,4) (6,5) (6,6)

A tree diagram is a device consisting of line segments emanating from a starting point and also
from outcome point. It is used to determine all possible outcomes of a probability experiment.

Example 2:
Find the sample space for the gender of the children if a family has three children. Use B for boy and G
for girl. Use a tree diagram.

Written by Cikgu_Marzuqi

Solution
Three are two genders, male and female, and each child could be either gender. Hence three are eight
possibilities, as shown here.
BBB BBG BGB GBB GGG GGB GBG BGG

Third Outcomes
Child
Second

First
Child B BBB
Child B
G BBG
B
B BGB
G
G BGG
GBB
B
G B
G GBG
G B GGB

G GGG

An event consists of set of outcomes of a probability experiment.
In general, a compound event consists of two or more outcomes or simple events.
There are basic interpretations of probability:
1. Classical probability
2. Empirical or relative frequency probability
3. subjective probability

Classical probability
Classical probability uses sample space to determine the numerical probability that an event will
happen. Classical probability assumes that all outcomes in the sample space are equally likely to
occur.
Equally likely events are event that have the same probability of occurring.


Formula for classical probability
P(E)=n(E)/n(S)

Example 3:
Since there are 4 suits (hearts, clubs, diamonds, and spades) and 13 cards for each suit (ace though king),
there are 52 outcomes in the sample space. A card is drawn from an ordinary deck. Find these
probabilities.
a) Of getting a jack
b) Of getting the 6 of clubs (i.e., a 6 and a club)
c) Of getting a 3 or a diamond
d) Of getting a 3 or 6
Solution
a) There are 4 jacks so there are 4 outcomes in event E and 52 possible outcomes in the sample space.
Hence, P(jack)= 4/52 = 1/13
b) Since there is only one 6 of clubs in event E, the probability of getting a 6 of clubs is
P(6 of clubs)= 1/52
c) There are four 3s and 13 diamonds, but the 3 of diamonds is counted twice in this listing. Hence, there
are 16 possibilities of drawing a 3 or a diamond, so P(3 or diamond)= 16/52 = 4/13
d) Since there are four 3s and 6s, P(3 or 6) = 8/52 = 2/13

Example 4:
When a single die is rolled, find the probability of getting a 9.
Solution
Since the sample space is 1, 2, 3, 4, 5, and 6, it is impossible to get a 9. Hence, the probability is
P(9) = 0/6 = 0

Example 5:
When a single die is rolled, what is the probability of getting a number less that 7?
Solution
Since all outcomes 1, 2, 3, 4, 5, and 6 are less than 7, the probability is P(number less than 7) = 6/6 = 1


Complementary events
The complement of an event E is the set of outcomes in the sample space that are not included
in the outcomes of event E. the complement of E is denoted by Ē
Example 6:
Find the complement of each event.
a. Rolling a die and getting a 4
b. Selecting a letter of the alphabet and getting a vowel
c. Selecting a month and getting a month that begins with a J
d. Selecting a day of the week and getting a weekday

Solution
a. Getting a 1, 2, 3, 5, or 6
b. Getting a consonant (assume y is a consonant)
c. Getting February, March, April, May, August, September, October, November, and December
d. Getting Saturday or Sunday

Rule for complementary event
P(Ē)=1-P(E) or P(E)=1-(Ē) or P(E)+P(Ē)=1

Example 7:
If the probability that a person lives in an industrialized country of the world is 1/5, find the probability
that a person does not live in an industrialized country.
Solution
P(not living in an industrialized country) = 1 – P(living in an industrialized country)
= 1 – 1/5 = 4/5
Empirical probability
The difference between classical and empirical probability is that a classical probability assume
that certain outcomes are equally likely, while empirical probability relies on actual experience
to determine the likelihood of outcome.

Formula for empirical probability
P(E) = f/n


Example 8:
In a sample of 50 people, 21 had type O blood, 22 had type A blood, 5 had type B blood, and 2 had type
AB blood. Set up a frequency distribution and find the following probabilities.
a. A person has type O blood.
b. A person has type A or type B blood.
c. A person has neither type A nor type O blood.
d. A person does not have type AB blood.
Solution
Type Frequency
A 22
B 5
AB 2
O 21
Total 50
a. P(O) = f/n = 21/50
b. P(A or B) = 22/50 + 5/50 = 27/50
(Add the frequencies of two classes)
c. P(neither A nor O) = 5/50 + 2/50 = 7/50
(Neither A nor O means that a person has either type B or type AB blood.)
d. P(not AB) = 1 – P(AB) = 1 – 2/50 = 48/50 = 24/25
(Find the probability of not AB by subtracting the probability of type AB from 1)

Example 9:
Hospital record indicated that knee replacement patients stayed in the hospital for the number of days
shown in the distribution.
Number of days stayed Frequency
3 15
4 32
5 56
6 19
7 5
127


Find these probabilities.
a. A patient stayed exactly 5 days. c. A patient stayed at most 4 days.
b. A patient stayed less than 6 days. d. A patient stayed at least 5 days.

Solution
a. P(5) = 56/127
b. P(fewer than 6 days) = 15/127 + 32/127 + 56/127 = 103/127
(Less than 6 days means 3, 4, or 5 days)
c. P(at most 4 days) = 15/127 + 32/127 = 47/127
(At most 4 days means 3 0r 4 days.)
d. P(at least 5 days) = 56/127 + 19/127 + 5/127 = 80/127
(At least 5 days means 5, 6, or 7 days.)

Subjective probability
Subjective probability uses a probability value based on an educated guess or estimate,
employing opinions and inexact information.

4.2 The addition rules for probability
Two event are mutually exclusive events if they cannot occur at the same time (i.e., they have
no outcomes in common)

Example 10:
Determine which events are mutually exclusive and which are not, what a single die is rolled.
a. Getting an odd number and getting an event number
b. Getting a 3 and getting an odd number
c. Getting an odd number and getting a number less than 4
d. Getting a number than 4 getting a number less than 4
Solution
a. The events are mutually exclusive, since the first event can be 1, 3, or 5 and the second event can be 2,
4, or 6.
b. The events are not mutually exclusive, since the first event is a 3 and the second can be 1, 3, or 5.
Hence, 3 is contained in both events.


c. The events are not mutually exclusive, since the first event can be 1, 3, or 5 and the second can be 1, 2,
or 3. Hence, 1 and 3 are contained in both events.
d. The events are mutually exclusive, since the first event can be 5 or 6 and h second event can be 1, 2,
or 3.

Addition rule 1
When two events A and B are mutually exclusive
P(A or B) = P(A) + P(B)

Example 11:
The corporate research and development center for three local companies have the following number
of employees:
U.S. Steel 110
Alcoa 750
Bayer Material Science 250
If a research employee is selected at random, find the probability that the employee is employed by U.S.
Steel or Alcoa.

Solution
P(U.S. Steel or Alcoa) = P(U.S. Steel) + P(Alcoa)
= 110/1110 + 750/1110
= 860/1110 = 86/111

Addition rule 2
If A and B are not mutually exclusive, then
P(A or B) = P(A) + P(B) – P(A and B)


Example 12:
In hospital unit there are 8 nurses and 5 physicians; 7 nurses and 3 physicians are females. If a staff
person is selected, find the probability that the subject is nurse or a male.

Solution
The sample space is shown here.
Staff Females Males Total
Nurses 7 1 8
Physicians 3 2 5
Total 10 3 13

The probability is
P (nurses or males) = P (nurse) + P (male) – P (male nurse)
= 8/13 + 3/13 – 1/13
= 10/13

4.3 The multiplication rules and conditional probability
The multiplication rules can be used to find the probability of two or more events that occur in
sequence.
Two events A and B are independent events if the fact that A occurs does not affect the
probability of B occurring.

Multiplication rule 1
When two events are independent, the probability of both occurring is
P (A and B) = P(A) • P(B)

Example 13:
An urn contains 3 red balls, 2 blue balls, and 5 white balls. A ball is selected and its color noted. Then it is
replaced. A second ball is selected and its color noted. Find the probability of each of these.
a. Selecting 2 blue balls
b. Selecting 1 blue ball and then 1 white ball
c. Selecting 1 red ball and then 1 blue ball


Solution
a. P(blue and blue) = P(blue) • P(blue) = 2/10 • 2/10 = 4/100 = 1/25
b. P(blue and white) = P(blue) • P(white) = 2/10 • 5/10 = 10/100 = 1/10
c. P(red and blue) = P(red) • P(blue) = 3/10 • 2/10 = 6/100 = 3/50

Multiplication rule 2
When two events dependent, the probability of both occurring is,
P(A and B) = P(A) • P(B/A)

Example 14:
At a university in western Pennsylvania, there were 5 burglaries reported in 2003, 16 in 2004, and 32 in
2005. If a researcher wishes to select at random two burglaries to further investigate, find the
probability that both will have occurred in 2004.

Solution
In this case, the events are dependent since the researcher wishes to investigate two distinct cases.
Hence the first case is selected and not replaced.
P(C₁ and C₂) = P(C₁) • P(C₁/C₂) = 16/53 • 15/52 = 60/689

Conditional probability
The conditional probability of an event B in relationship to an event A was defined as the
probability that event B occurs after event A has already occurred.

Formula for conditional probability
P(B/A) = P(A and B) / P(A)


Example 15:
A recent survey asked 100 people if they thought women in the armed forces should be permitted to
participate in combat. The results of the survey are shown.

Gender Yes No Total
Male 32 18 50
Female 8 42 50
Total 40 60 100

Find the Probabilities.
a. The respondent answered yes, given that the respondent was a female.
b. The respondent was a male, given that the respondent answered no.
Solution
Let M = respondent was a male Y = respondent answered yes
F = respondent was a female N = respondent answered no
a. The problem is to find P(Y/F). The rule states
P(Y/F) = P(F and Y) / P(F)
The probability P(F and Y) is the number of female who responded yes, divided by the total number of
respondents:
P(F and Y) = 8/100
The probability P(F) is the probability of selecting a female:
P( F) = 50/100
Then
P(Y/F) = P(F and Y) / P(F)
= 8/100 ÷ 50/100
= 4/25
b. The problem is to find P(M/N)
P(M/N) = P(N and M) / P(N)
= 18/100 ÷ 60/100
= 3/10


Probability for “At Least”
Example 16:
A coin is tossed 5 times. Find the probability of getting at least 1 tail.
Solution
It is easier to find the probability of the complement of the event, which is “all heads,” and then subtract
the probability from 1 to get the probability of at least 1 tail.
P(E) = 1 – P (Ē)
P(at least 1 tail) = 1 – P(all heads)
P(all heads) = (1/2)⁵ = 1/32
Hence, P(at least 1 tail) = 1 – 1/32 = 31/32

4.4 Counting rules
To determine this number, three rules can be used; the fundamental counting rule, the
permutation rule, and the combination rule.

Fundamental counting rule
k₁ • k₂ • k₃ • • • • kn

Example 17:
The manager of a department store chain wishes to make four-digit identification cards for her
employees. How many different cards can be made if she uses the digits 1, 2, 3, 4, 5, and 6 and
repetitions are permitted?

Solution
Since there are 4 spaces to fill on each card there are 6 choices for each space, the total number of cards
that can be made is 6 • 6 • 6 • 6 = 1296

Factorial Formulas
Permutations
n! = n(n - 1)(n - 2) ••• 1
A permutation is an arrangement of n objects in a specific order.
0! = 1

Permutation rule
nP r = n! / (n-r)!


Example 18:
A school musical director can select 2 musical plays to present next year. One will be presented in the
fall, and one will be presented in the spring. If she has 9 to pick from how many different possibilities are
there?
Solution
Order is important since one play can be presented in the fall and the order play in the spring.
₉P₂ = 9! / (9-2)!
= 9! / 7! = 9 •8 • 7! / 7!
= 72
There are 72 different possibilities.

Combination rule
nCr = n! / (n-r)! r!

Example 19:
In a club there are 7 women and 5 men. A committee of 3 women and 3 men is to be chosen. How many
different possibilities are there?
Solution
₇C₃ • ₅C₂ = 7! / (7-3)! 3! • 5! / (5-2)! 2!
= 350


4.5 Probability and counting rule
Example 20:
A box contains 24 transistors, 4 of which are defective. If 4 are sold at random, find the following
probabilities.
a. Exactly 2 are defective. c. All are defective.
b. None is defective. d. At least 1 is defective.

Solution
There are ₂₄C₄ ways to sell 4 transistors, so the denominator in each case will be 10,626.
a. Two defective transistors can be selected as ₄C₂ and two non-defective ones as ₂₀C₂. Hence,
P(exactly 2 defectives) = ₄C₂ •₂₀C₂ / ₂₄C₄
= 1140/10,626 = 190/1771
b. The number of ways to choose no defectives is ₂₀C₄. Hence,
P(no defectives) = ₂₀C ₄/ ₂₄C₄
= 4845 / 10,626 = 1615 / 3542
c. The number of ways to choose 4 defectives from 4 is ₄C₄, or 1. Hence,
P(all defective) = 1 / ₂₄C₄
= 1 / 10,626
d. To find the probability of at least 1 defective transistor, find the probability that there are no
defective transistors, and then subtract that probability from 1.
P(at least 1 defective) = 1 – P(no defectives)
= 1 – ₂₀C₄ / ₂₄C₄
= 1 - 1615 / 3542 = 1927 / 3542


Probability and counting rules

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