The average weekly earnings of bus drivers in a city are $950 (that is m) with a standard deviation of $45 (that is s). Assume that we select a random sample of 81 bus drivers.

a. Compute the standard error of the mean.

b. What is the probability that the sample mean will be greater than $960?

c. If the population of bus drivers consisted of 400 drivers, what would be the standard error of the mean?