Wafers of type A have thicknesses with a mean of 134.9 and a standard deviation of 0.7, while wafers of type B have thicknesses with a mean of 138.2 and a standard deviation of 1.1. The thicknesses of the wafers are all independent of each other.
(a) Three wafers of type A are stacked on top of each other. What are the mean and the standard deviation of the total thickness?
(b) Two wafers of type A and two wafers of type B are stacked on top of each other. What are the mean and the standard deviation of the total thickness?
(c) Four wafers of type A and three wafers of type B are taken. What are the mean and the standard deviation of the average thickness of these seven wafers?