Refer to the situation of problem 8-72. The study also compared average portfolios of microloans. For the Alpha group it was $50 million, and for the Beta group it was $14 million. Assume the Alpha group had a standard deviation of $20 million and the Beta group had a standard deviation of $8 million. Construct a 95% confidence interval for the difference in mean portfolio size for firms in the two groups of credit ratings.