standard deviation
A service consists of two parts. The first part takes an average of 10 minutes with a standard deviation of 2 minutes; the second part takes an average of 5 minutes with a standard deviation of 1 minute. Describe how long it takes to complete the service. What is the probability that a customer can be served in less than 12 minutes? What is the probability that service to a customer will take more than 20 minutes?
standard deviation