18. Suppose that the distribution of scores on the Graduate Record Exam (GRE) is approximately normal, with a mean of \( \mu=150 \) and a standard deviation of \( \sigma=5 \). For the population of students who have taken the GRE: a. What proportion have GRE scores less than 145 ? b. What proportion have GRE scores greater than 157 ? c. What is the minimum GRE score needed to be in the highest \( 20 \% \) of the population? d. If a graduate school accepts only students from the top \( 10 \% \) of the GRE distribution, what is the minimum GRE score needed to be accepted?
