A car manufacturer is testing an engine model that they believe will yield a different highway mpg than the average car currently on the market. To test this theory, they randomly sample 50 cars with the new engine model and yields an average mpg of 26.8 highway mpg with a standard deviation of 4.5 mpg. According to recent data, the current average is 25.1 highway mpg. Using an alpha of 0.05, what is the critical value?