4.38 The average playing time of compact discs in a large collection is 35 minutes, and the standard deviation is 5 minutes.

a. What value is 1 standard deviation above the mean? 1 standard deviation below the mean?

What values are 2 standard deviations away from the mean? (Hint: See Example 4.14.)

b. Without assuming anything about the distribution of times, at least what percentage of the times is between 25 and 45 minutes? (Hint: See Example 4.15.)

c. Without assuming anything about the distribution of times, what can be said about the percentage of times that are either less than 20 minutes or greater than 50 minutes?

d. Assuming that the distribution of times is approximately normal, about what percentage of times are between 25 and 45 minutes? less than 20 minutes or greater than 50 minutes? less than 20 minutes?