Confusion:You and I

I have three chinese friends;Ai,Yu and Mi.Ai is good at maths,Yu is doing Art and Mi is doing French.I said,'I don't understand calculus' (facing Ai) I said,'Will you teach me?'.Ai interjected,'I know calculus' and Yu said,'I can't teach you what I don't know' Mi replied,'Of what use is calculus to me?'