Logic Riddles

Question: Keys! keys! So many keys, When F is involved The key note is _ ?

Question: You have 20 sick sheep, one dies. How many are left?

Question: What does this code mean?: YY U R YY U B I C U R YY 4 ME

Question: Two brothers are twins. One twin says he is older than the other. The other twin is confused and says: in order for us to be twins we have to born on the same day. How is the one twin older than the other?

Question: There are three books on a shelf standing side by side. Each book is 10ml thick. The bookworm eats its way through the first page of the first book to the last page of the last book. How far did the bookworm move if it can only move in a straight horizontal line?