The operations manager of a vehicle manufacturing plant in London wants to estimate the average size of an order received. An order is measured by the number of vehicles shipped. A random sample of 100 orders from dealerships had a sample mean value of 135 vehicles. Assume that the population standard deviation is 30 vehicles and that the order size is normally distributed.
Question: Estimate, with a 90% confidence, the mean size of orders received from all the manufacturer's customers (use the z - table, round off to the nearest whole number).