Expected demand (mean) = 20,000 units Probability range for demand = 10,000 units to 30,000 units (95% probability) To approximate a normal distribution, we can use the mean and standard deviation. The standard deviation can be calculated using the probability range (z-scores) and the formula for standard deviation (): From the standard normal distribution table, for a 95% confidence interval, the z-scores are approximately -1.96 and 1.96. calculate z score