Exercise7_setup
exercise7_1 - print student hash
exercise7_2 - add a new cohort key,value
Exercise7_3 - output all keys from the hash
Exercise7_4 - add 5% to each class (you can't have fraction of a student)
Exercise7_5 - delete the 2nd cohort w/out knowing the key or value
Exercise7_6 - sum all students in each cohort, but only last hash set