Question

Marco has a jug with 3 7/2 ounces of juice. he wants to use some small cups to pass out samples of the juice

Answer 1

If the jug holds only 3 7/2 ounces [13/2 ounces) of juice, the number of 2 1/2 oz. cups that can be filled is

13
----
  2
-------  =  13/5 cups, or 2 3/5 cups.
 5
---
 2

I suspect you have copied down your numbers incorrectly.  Try again.