Suppose that the impurity levels of water samples taken from a particular source are independent with a mean value of 3.87 and a standard deviation of 0.18.
(a) What are the mean and the standard deviation of the sum of the impurity levels from two water samples?
(b) What are the mean and the standard deviation of the sum of the impurity levels from three water samples?
(c) What are the mean and the standard deviation of the average of the impurity levels from four water samples?
(d) If the impurity levels of two water samples are averaged, and the result is subtracted from the impurity level of a third sample, what are the mean and the standard deviation of the resulting value?