The following frequency distribution presents a set of exam scores for a class of N = 20 students.
x f cf c%
90-99.....4 ... 20... 100
80-89.... 7 .....16..... 80
70-79 ....4 .......9...... 45
60-69.... 3....... 5....... 25
50-59..... 2...... 2......... 10
a) Find the 30th percentile.
b) What is the percentile ranking for X=77?