Two operators perform the same machining operation. Their supervisor wants to estimate the difference in the mean machining times between them. No assumption can be made as to whether the variabilities of machining time are the same for both operators. It can be assumed, however, that the distribution of machining time is normal for each operator. A random sample of 10 from the first operator gives an average machining time of 4.2 min with a standard deviation of 0.5 min. a random sample of 6 from the second operator yields an average machining time of 5.1 min with a standard deviation 0.8 min. Find a 95% confidence interval for the difference in the mean machining times between the two operators.