This is a play on words! The riddle goes: "There are 30 cows in the field, and 28 chickens. How many didn't?"
The trick is in the phrasing: "28 chickens" sounds like "twenty ate chickens." So, if 20 cows ate chickens, that leaves 10 cows that didn't eat any.
Therefore, the answer is 10 cows didn't eat chickens.
