I have a chocolate bar and I want to give 10% to my brother, how much should i give him? (hershey)?

Go into another room where he's not watching. Break off a corner of the chocolate. Eat the rest. Then give the piece to your brother and say that it's exactly 10% and that if he has to take it or leave it, and if he leaves it, then it's the same as him giving the piece back to you, and you'll eat it. If he raises any objections, just eat the piece. That will teach him a lesson.