On a particular day, 50% of the employees in a large company had arrived at work by 8.30 a.m., and 10% had not arrived by 8.55 a.m.

(a) Assuming a normal model, find the standard. deviation of the arrival times, in minutes.

(b) It is given that only 5% of the employees had arrived by 8.05 a.m. Without further calculation, explain why this might suggest that a normal model is not appropriate.

(c) Eighty employees are selected at random. Find the expectation of the number of these employees that arrived between 8.30 and 8.55 a.m. (C)