These 3 students who left have each 13 points above 65. Combine they have weighting of 13X 3= 39 points that pull up the average to 65.
Now these 39 points is lost when the 3 students leave. The effect of the 39 points loss is shared among the remaining students such that each student who was an average of 65 dropped by 3 points to 62 points.
To summarise the above statement, the 3 points dropped each is that burden they have to bear for the 39 points lost.
Number of children who bear the drop of 3 points each = 39/3 = 13 students
Originally would be 16 if you add in those three 78 pointer students.
Working wise is actually very simple if you DC understand the above concept, its just these few lines below,
78- 65 = 13
Total points loss = 13 X 3 =39
Number of students left = 39/3 = 13
Originally number of students = 13 + 3 = 16 ###