5.2, 8.3, 8.5, 7.7, 7.8, y; mean = 7.1

You know that to find the mean you add all the numbers and divide by how many numbers there are. So work backwards.

add all the numbers you have=37.3

Since you divided by 6, do the opposite 7.1 x 6 = 42.6

Now subtract the difference = 42.6 - 37.3 = 5.5