The riddle is a play on words:

"There are 30 cows in a field, and 28 chickens. How many didn't?"

If you read "28" as "twenty-eight," it seems confusing. However, if you read it as "twenty ate," the riddle becomes clearer.

So, there are 30 cows, and 28 (twenty ate) chickens. This means 20 cows ate chickens, leaving 10 cows that didn't.

Therefore, 10 cows didn't eat chickens.