Owing to the variability of trade-in allowance, the profit per new car sold by an automobile dealer varies from car to car. The profits per sale (in hundreds of dollars), tabulated for the past week, were 2.1, 3.0, 1.2, 6.2, 4.5, and 5.1. Find a 90% confidence interval for the mean profit per sale. What assumptions must be valid for the technique that you used to be appropriate?