According to the American College Test (ACT), results from the 2004 ACT testing found that students had a mean reading score of 21.3 with a standard deviation of 6.0. Assuming that the scores are normally distributed:
a. Find the Probability that a randomly selected student has a reading ACT score less than 20.
b. Find the probability that a randomely selected student had a reading ACT score between 18 and 24.
c. Find the probability that a randomly selected student has a reading score ACT score greater than 30.
d. Find the value of the 75th percentile for ACT scores.