Assigning grades

You are given a dataframe (called df) of students, and you need to assign grades to each student. The table schema is as follows:

Column name	Data type	Example value	Description
student_name	string	Cindy Chu	Name of student
student_id	integer	1837204	Unique id for each student
class	string	Biology	Name of class
final_grade_pct	integer	80	final grade for class represented in percent

You need to assign the following letter grades based on final_grade_pct in a new column named "final_grade_letter":

• 90 - A

• 81-90 - B
• 71-80 - C
• <70 - D

Write a function using Python to loop through the table and assign the appropriate letter grades to each student, adding a new column to the existing dataframe, df.