A cereal manufacturer is concerned that the boxes of cereal not be under filled or overfilled. Each box of cereal is supposed to contain 13 ounces of cereal. A random sample of 36 boxes is taken and the mean weight is found to be 12.8 ounces with a standard deviation of 0.5 ounces. What is the probability that the sample mean weight is less than 12.9 ounces?