Question: To assess to the relative happiness of men and women in their marriages, a marriage counselor plans to administer a test measuring happiness in marriage to n randomly selected married couples, record the their test scores, find the differences, and then draw inferences on the possible difference. Let μ1 and μ2 be the true average levels of happiness in marriage for men and women respectively as measured by this test. Suppose it is desired to find a 90% confidence interval for estimating μd = μ1 - μ2 to within two test points. Suppose further that, from prior studies, it is known that the standard deviation of the differences in test scores is σd ≈ 10. What is the minimum number of married couples that must be included in this study?