in answer to the 4 people with the wall between riddle #1 sees a white and a black hat in front of him. He cannot tell what hat he is wearing, so he keeps his mouth shut. If #1 saw two black hats, he would know that he was wearing a white hat. Because #1 does not say anything, #2 knows that #1 must see a black and a white hat. #2 sees a black hat and therefor he knows that he himself wears a white hat. Thats the version i answered.