Environmental impact study. Some power plants are located near rivers or oceans so that the available water can be used to cool the condensers. Suppose that, as part of an environmental impact study, a power company wants to estimate the difference in mean water temperature between the discharge of its plant and the offshore waters.

How many sample measurements must be taken at each site in order to estimate the true difference between means to within .3°C with 90% confidence? Assume that the range in readings will be about 4°C at each site and that the same number of readings will be taken at each site.