If cells are: a1=90, a2=85, a3=80, a4=75, and a5=75. what will be your formula if you are going to get the average?

The average can be given by the following formula:
 Average = (a1 + a2 + a3 + a4 + a5) / (5).
 Equivalently.
 Average = (ai) / (i).
 Where,
 i = 1,2,3,4, ..., n
 Substituting
 Average = (90 + 85 + 80 + 75 + 75) / (5).
 Average = 81
 
 Average = (a1 + a2 + a3 + a4 + a5) / (5).
 or
 Average = (ai) / (i).
 Where,
 i = 1,2,3,4, ..., n