Chapter 6
Day 3
Warm-Up
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


A sociologist studies social mobility in England by
recording the social class of a large sample of fathers
and their sons. The social classes are ordered from Class
1 (lowest) to Class 5 (highest). Here are the probabilities
that the son of a lower class (Class 1) father will end up
in each social class.
Son’s Class
1
2
3
4
5
Probability
.48
.38
.08
.05
.01
Let A be the event that the son remains in class 1
Let B be the event that the son reaches one of the two
highest classes.
Find the following probabilities




P(A)
P(B)
Probability that the son does NOT remain in class 1? P(A c)
Probability that the son remains in the lower class or reaches
one of the top two classes. P(A or B)
Homework Solutions #7-13
7.
A. 0.04
B. 0.69
8. 0.54
9. 0.67 due to cardiovascular disease or cancer
0.33 due to other causes
10. A. 0.27
B. 0.73
11. A. 0.65
B. 0.38
C. 0.62
12. A. 1
B. 0.59
C. 0.64
13. Win a large battle 0.6 probability vs. 0.512
probability of winning three small battles
 Multiplication
as well!
 Look
rules work for compliments
at example #15
 A string of Christmas lights contains 20
lights. The lights are wired in a series, so
that if any light fails the whole string will go
dark. Each light has a probability or 0.02
of failing during a 3-year period. The lights
fail independently of each other. What is
the probability that the string of lights will
remain bright for 3 years?
 What
is the probability of each light not
failing?
 What
is the probability of all 20 lights not
failing?
Example
 Most
sample surveys use random digit
dialing equipment to call residential
telephone numbers at random. The
telephone polling firm Zogby International
reports that the probability that a call
reaches a live person is 0.2. Calls are
independent.
 A polling firm places 5 calls. What is the
probability that none of them reaches a
person?
 When calls are made to NYC, the
probability of reaching a person is only
0.08. What is the probability that none of 5
calls made to NYC reaches a person?
Example (classwork)

The table below shows the probabilities of
selecting a given color of M&Ms:
Color
Probability
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


Brown
Red
Yellow
Blue
.20
.10
.20
.30
Does the table contain all colors? How do you
know?
What is the probability of choosing and M&M
of a different color than what is listed?
What is the probability of choosing an M&M
that is brown or yellow?
If you choose 2 M&Ms at random, what is the
probability that both are brown or yellow?