Annabelle from England asks, "A farmer had 30 cows and 28 chickens.how many are left?"

Well, Annabelle, I think you may be conflating two different riddles that are somewhat similar, because I've never heard that riddle stated like that. 

Here are the two riddles I have heard, which are similar:

Riddle One: A farmer has 26 sheep and 2 died. How many are left?

Riddle Two: A farmer has 30 cows and 28 chickens. How many didn't?

The problem is that neither of these riddles are fair if you put them in writing, because they rely on similarities of spoken words to trick the listener. Once you commit it to writing, it's no longer a fair question.

You see, in riddle number one, if I was asking it out loud, what I would really be saying is: "A farmer had twenty sick sheep, and two died." So clearly the answer is 18, instead of the 24 you were expecting.

In the second riddle (which sounds like it makes no logical sense), the question (when spoken) is actually "A farmer has 30 cows, and 20 ate chickens." So if there were 30, and 20 of them ate chickens, 10 didn't.

So there you go.  A couple riddles that aren't fair to commit to writing, but are fun if you speak them aloud!

And while we're on riddles, here's a riddle I told some people at my school, and then told them, "In five minutes you won't be able to tell this riddle to anyone."

The riddle is: "Do you know how long cows should be milked?"

And the answer is: "The same as short ones."

Five minutes later I asked them to repeat the riddle back to me, and I got things like:

"How long should you milk a cow?"
"How do you milk a long cow?"
"How are long cows milked?"

It's a fun riddle, because if you don't word it exactly the way I did originally, it spoils the joke!