A random sample of 10 sales receipts for internet sales results in a mean sale amount of $66.30 with a standard deviation of $15.75. Using this data, find the 98% confidence interval for the true mean difference between the mean amount of mail-order purchases and the mean amount of internet purchases. Assume that the population variances are not equal and that the two populations are normally distributed. Step 1 of 3 : Find the point estimate that should be used in constructing the confidence interval.