A standardized​ exam's scores are normally distributed. In a recent​ year, the mean test score was 21.4 and the standard deviation was 5.6. The test scores of four students selected at random are 15​, 23​, 8​, and 36. Find the​ z-scores that correspond to each value and determine whether any of the values are unusual. The​ z-score for 15 is nothing.