Suppose you work as the qualoty control officer of a factory producing vaccum cleaners. You want to estimate the duration of trouble-free operation of a new vaccum cleaner. The duration of trouble-free operation is normally distributed with a standard deviation of 300 hours. You select a random sample of 25 vacuum cleaners and test them until they show the first signs of performance problems. The mean trouble-free duration for this sample is 2650 hours.