-10grade/week
Explanation:
Given parameters: 
Initial grade = 95grade
Final grade = 65grade
Time = 3weeks
Unknown parameter: 
Rate of change of grade = ? 
Solution: 
Rate is defined as the change of a quantity with time. 
  Rate of change of grade  = 
Rate of change of grade  =  =
 = 
Rate of change of grade = -10grade/week
The rate of change of the student's grade is -10grade/week. It implies that the grade of the student reduces by 10unit per week.