Statistics for the Behavioral Sciences by Frederick J. Gravetter, Larry B. Wallnau ISBN 10: 1305504917 ISBN 13: 9781305504912

PROBLEMS 191
19. A report in 2010 indicates that Americans between the
ages of 8 and 18 spend an average of μ = 7.5 hours
per day using some sort of electronic device such as
smart phones, computers, or tablets. Assume that the
distribution of times is normal with a standard deviation
of σ = 2.5 hours and find the following values.
a. What is the probability of selecting an individual who
uses electronic devices more than 12 hours a day?
b. What proportion of 8- to 18-year-old Americans
spend between 5 and 10 hours per day using electronic
devices? In symbols, p(5 < X < 10) = ?
20. Rochester, New York, averages μ = 21.9 inches of
snow for the month of December. The distribution
of snowfall amounts is approximately normal with a
standard deviation of σ = 6.5 inches. A local jewelry
store advertised a refund of 50% off all purchases
made in December, if we have more than 3 feet
(36 inches) during the month. What is the probability
that the jewelry store will have to pay off on its
promise?
21. A multiple-choice test has 32 questions, each with
four response choices. If a student is simply guessing
at the answers,
a. What is the probability of guessing correctly for
any individual question?
b. On average, how many questions would a student
answer correctly for the entire test?
c. What is the probability that a student would get
more than 12 answers correct simply by guessing?
22. A true/false test has 20 questions. If a student is
simply guessing at the answers,
a. On average, how many questions would a student
answer correctly for the entire test?
b. What is the probability that a student would answer
more than 15 questions correctly simply by guessing?
c. What is the probability that a student would
answer fewer than 7 questions correctly simply by
guessing?
23. A roulette wheel has alternating red and black,
numbered slots into which the ball finally stops to
determine the winner. If a gambler always bets on
black to win, then
a. What is the probability of winning at least 40 times
in a series of 64 spins? (Note that at least 40 wins
means 40 or more.)
b. What is the probability of winning more than
40 times in a series of 64 spins?
c. Based on your answers to a and b, what is the
probability of winning exactly 40 times?
24. A test developed to measure ESP involves using Zener
cards. Each card shows one of five equally likely
symbols (square, circle, star, cross, wavy lines), and
the person being tested has to predict the shape on
each card before it is selected. Find each of the probabilities
requested for a person who has no ESP and is
just guessing.
a. What is the probability of correctly predicting
exactly 20 cards in a series of 100 trials?
b. What is the probability of correctly predicting 10
or more cards in a series of 36 trials?
c. What is the probability of correctly predicting
more than 16 cards in a series of 64 trials?
25. The student health clinic reports that only 30% of students
got a flu shot this year. If a researcher surveys
a sample of n = 84 students attending a basketball
game,
a. What is the probability that any individual student
has had a flu shot?
b. What is the probability that more than 30 students
have had flu shots?
c. What is the probability that 20 or fewer students
have had shots?