The test scores for the verbal reasoning and the quantitative reasoning sections of the Graduate Record Examination (GRE) are normally distributed. In a recent year, the mean test score was 150 and the standard deviation was 8.75. The test scores of four students selected at random are 162, 168, 155, and 138.

(a) Find the z-score that corresponds to each value and 

(b) Determine whether any of the values are unusual.