16, 6, 68, 88, __, 98

We turn that upside-down (or stand on our heads), and see this:
     __
86 ’   ’88 ’89 ’9 ’9I

I think you meant to have a 0 before the 6, like this

16, 06, 68, 88, __, 98

Then if we turn it upside down (or stand on our heads, we see this:
     __ 
86 ’   ’88 ’89 ’90 ’9I

So obviously 87 ought to go between 86 and 88

    
86 ’87 ’88 ’89 ’90 ’9I

Then we turn it back right-side up again, and we have:

I6, 06, 68, 88, L8, 98

Answer: L8   J

Edwin