Suppose that the distribution of marks on an exam is closely described by a normal curve with a mean of 65. The 84th percentile of this distribution is 75. (a) What is the 16th percentile? (b) What is the approximate value of the standard deviation of exam marks? (c) What z score is associated with an exam mark of 50? (d) What percentile corresponds to an exam mark of 85? (e) Do you think there were many marks below 35? Explain. These are some exercises the prof gave us. That's all of the information given. I'm having some difficulty getting started on a and b particularly.