It's the kind of riddle that only works if you speak it; when written out it's not the same question.
What we hear is: "50 cows in a field"...
We hear "28 chickens"...
but the person speaking is actually saying, "Twenty ate chickens"...
Then, "How many didn't?"
Since cows don't actually eat chickens...well...
(When I first learned the riddle there were only 30 cows in the field: 30 - 20- = 10, hence my incorrect answer earlier.)
