This isn't as hard as it seems if we can write them in a little different mannner.....
81/2 = [23]1/2 = 23/2 = 29/6
42/3 = [22]2/3 = 24/3 = 28/6
321/3 = [25]1/3 = 25/3 = 2 10/6
25/6 = [2]5/6 = 25/6
And since 10/6 > 9/6 > 8/6 > 5/6
Then 2 10/6 > 29/6 > 28/6 > 25/6
So....in ascending order, we have
25/6 , 42/3, 81/2, 321/3