Answer:
15.
To solve this, we can use little hints in the question.
For instance, rather than "chickens," you used "chicken."
Also, you used 35 for the cows, but then spelled out 28.
That's a bit odd, so it has to mean something.
So, let's see if there are any double meanings or hidden words.
Well, twenty-eight sounds like twenty ate.
twenty ate chicken
that makes sense unless it's something else and I'm confused.