+128475 Let x be the missing value we need
Average (mean) = [ sum of the values ] / [ number of values ....so we have....
80 = [ 71 + 91 + 82 + 90 + 88 + 61 + 70 + x ] / 8
80 = [553 + x ] / 8 multiply both sides by 8
640 = 553 + x subtract 553 from both sides
87 = x = the missing value
