5.35 A company services copiers. A review of its records shows that the time taken for a service call can be represented by a normal random variable with a mean of 75 minutes and a standard deviation of 20 minutes.

a. What proportion of service calls takes less than 1 hour?

b. What proportion of service calls takes more than 90 minutes?

c. Sketch a graph to show why the answers to parts

(a) and

(b) are the same.

d. The probability is 0.1 that a service call takes more than how many minutes?