Mean of a set of numbers is the quotient when their sum is divided by the number of items in the set. For the given above, their sum (S) is,
S = 5.2 + 8.3 + 8.5 + 7.7 + 7.8 + y = 37.5 + y
The mean (m) is calculated through the formula,
m = S / n
where n is the number of items. In this given there are 6 items including y.
7.1 = (37.5 + y) / 6 ; y = 5.1
Therefore, y is equal to 5.1.