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Chapter 5: Q .80. (page 338)
Sampling students A statistics class with 30 students has 10 males and 20 females. Suppose you choose 3 of the students in the class at random. Find the probability that all three are female.
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Four-sided dice A four-sided die is a pyramid whose four faces are labeled
with the numbers and(see image). Imagine rolling two fair, four-sided dice and
recording the number that is showing at the base of each pyramid. For instance, you would
record a if the die landed as shown in the figure.
a. Give a probability model for this chance process.
b. Define event A as getting a sum of . Find P(A).
Crawl before you walk (At what age do babies learn to crawl? Does it take
longer to learn in the winter, when babies are often bundled in clothes that restrict their
movement? Perhaps there might even be an association between babies鈥� crawling age and
the average temperature during the month they first try to crawl (around months after
birth). Data were collected from parents who brought their babies to the University of
Denver Infant Study Center to participate in one of a number of studies. Parents reported
the birth month and the age at which their child was first able to creep or crawl a distance
of feet within one minute. Information was obtained on infants. Crawling age is given in weeks, and average temperature (in degrees Fahrenheit) is
given for the month that is months after the birth month.
a. Make an appropriate graph to display the relationship between average temperature and
average crawling age. Describe what you see.
Some computer output from a least-squares regression analysis of the data is shown.
b. What is the equation of the least-squares regression line that describes the relationship
between average temperature and average crawling age? Define any variables that you
use.
c. Interpret the slope of the regression line.
d. Can we conclude that warmer temperatures months after babies are born causes them
to crawl sooner? Justify your answer.
Middle school values Researchers carried out a survey of fourth-, fifth-, and sixth-grade students in Michigan. Students were asked whether good grades, athletic ability, or being popular was most important to them. The two-way table summarizes the survey data. 
Suppose we select one of these students at random. What鈥檚 the probability of each of the following?
a. The student is a sixth-grader or rated good grades as important.
b. The student is not a sixth-grader and did not rate good grades as important.
Genetics There are many married couples in which the husband and wife both carry a gene for cystic fibrosis but don鈥檛 have the disease themselves. Suppose we select one of these couples at random. According to the laws of genetics, the probability that their first child will develop cystic fibrosis is
a. Interpret this probability as a long-run relative frequency.
b. If researchers randomly select such couples, is one of these couples guaranteed to have a first child who develops cystic fibrosis? Explain your answer.
Foreign-language studyChoose a student in grades 9 to 12 at random and ask if he or she is studying a language other than English. Here is the distribution of results:
a. What鈥檚 the probability that the student is studying a language other than English?
b. What is the probability that a student is studying Spanish given that he or she is
studying some language other than English?
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