Answer:
We will first find out average time of each student.
Student 1 = 60 / 12 = 5
Student 2 = 60 / 10 = 6
Student 3 = 60 / 8 = 7.5
Student 4 = 60 / 6 = 10
Student 5 = 60 / 4 = 15
Now we will take the arithmatic mean of all students average time.
A = (5+6+7.5+10+15)/5 = 43.5/5 = 8.7 minutes per problem