Erin teaches beginner dance classes to young children on Monday - Thursday. She has 9 advanced students who are available to assist with the younger students. She needs to determine how many assistants should work on each day so that the younger children can all have access to as much individual attention as possible.

Day Students
Mondays 15
Tuesdays 19
Wednesdays 12
Thursdays 18


What will be the standard divisor for her calculations?