13 packs of meat patties and 16 packs of buns
(13 + 20) + 15 = 13 + (20 + 15)
Since
and
, we have

So, the equation becomes

The solutions to this equation are 4 and -5/2, so I'm afraid there's a typo in your question.
Answer:
D. 9/DF=18/16.......the answer