The phrasing of your question appears to be a riddle or a play on words rather than a straightforward math problem. It seems like you're asking for a clever or humorous answer rather than a numerical one.
If you're looking for a riddle-like interpretation, it could be something like "How many total animals are there?" which would mean adding the cows and chickens together:
20 cows + 28 chickens = 48 animals.
However, if you're referring to the riddle that goes "There are 20 cows on a farm and 28 chickens," the answer plays on the words: "28" sounds like "twenty ate." So, the joke would be that "20 cows ate chickens."
Let me know if you'd like any clarification!