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A geology teacher takes her class on an annual field trip to analyze rock samples. They use randomized sets of
coordinates to select samples from two regions.
Suppose they examine a sample of 70 rocks from region A, which is composed of 15% igneous rock. Then they
examine a sample of 60 rocks from region B, which is composed of 20% igneous rock. Then they look at the
difference between the sample proportions (PA - B).
What are the mean and standard deviation of the sampling distribution of pA - PB?