A manufacturer of a gas additive claims that it improves gas mileage. A random samples of 30 drivers tests this claim by determining their gas mileage for a full tank of gas that contains the additive (X1), and the full tank of gas that does not contain the additive (X2). The sample mean difference, D, equals 2.12 miles (in favor of the additive), and the estimated standard error equals 1.50 miles