A major car manufacturer wants to test a new engine to determine if it meets new air pollution standards. Safety regulations require that the mean emission of all engines of this type must be no greater than 20 parts per million (ppm) of carbon. If it is higher than that, they will have to redesign parts of the engine. A random sample of 25 engines is tested and the emission level of each is determined. The sample mean is calculated to be 20.4 ppm and the sample standard deviation is calculated to be 2.0 ppm. It is known that emission levels are normally distributed with standard deviation 1.6 ppm. We would like to test whether the true mean emission level for the new engine is greater than 20 ppm. The test statistic for the appropriate test of significance is: