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?