In a random sample of 310 cars driven at low altitudes, 41 of them exceeded a standard of 10 grams of particulate pollution per gallon of fuel consumed. In an independent random sample of 95 cars driven at high altitudes, 21 of them exceeded the standard. Compute the test statistic for testing if the proportion of high-altitude vehicles exceeding the standard is greater than the proportion of low-altitude vehicles exceeding the standard.