Hey,

A company has moved to new premises and the management have reason to believe that the number of internal telephne calls will increase. At the old premises the daily number of calls could be described by a normal distribution with a mean of 110. For 5% of the days, the number of calls was either less than 80 or above 140.

Question 1: Calculate the standard deviation.

How do I do that?

Knowing that the area of under the bell curve above 140 or below 80 is equal to 5%, you can look up the "z-score" that corresponds to either 5% or 95% (because you know the area to the left of 80 is 5%, and the area to the left of 140 is 95%). That will tell you how many standard deviations 80 and 140 are away from the mean of 110. Suppose the z-score is 2 (it's not, but just suppose). Then you know that one standard deviation is half of 140-110. Hope this helps.