8) What proportion of a normal distribution is located betweeneach of the following z-score boundaries?
a. z = –0.25 and z = +0.25
b. z = –0.67 and z = +0.67
c. z = –1.20 and z = +1.20
13) A normal distribution has a mean of ? = 30 and a standarddeviation of ? = 12. For each of the following scores, indicatewhether the body is to the right or left of the score and find theproportion of the distribution located in the body.
a. X = 33
b. X = 18
c. X = 24
d. X = 39
19) A report in 2010 indicates that Americans between the agesof 8 and 18 spend an average of ? = 7.5 hours per day using somesort of electronic device such as smart phones, computers, ortablets. Assume that the distribution of times is normal with astandard deviation of ? = 2.5 hours and find the followingvalues.
a. What is the probability of selecting an individual who useselectronic devices more than 12 hours a day?
b. What proportion of 8- to 18-year-old Americans spend between5 and 10 hours per day using electronic devices? In symbols, p (5< X < 10) = ?
