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 answer: Average = (a1 + a2 + a3 + a4 + a5) / (5). or Average = (ai) / (i). Where, i = 1,2,3,4, ..., n