Assume that the amount of time (in minutes) for eighth graders to compete an assessment examination is 78 minutes with a standard deviation of 12 minutes.
a. What proportion of eighth graders completes the assessment examination in 72 minutes or less?
b. What proportion of eighth graders completes the assessment examination in 82 minutes or more?
c. For what number of minutes would 90% of all eighth graders complete the assessment examination?