[Kou takes the notebook. He follows the problem in order and comes to what is simply a gaping hole. He doesn't fathom how Kai could've missed the whole thing unless he fell asleep, as said.]
[He looks appraisingly at Kai for a moment before explaining.]
You look like a person who doesn't sleep properly. If you did, this wouldn't be a problem.
[He is concerned. He's just got a ways to go before he feels more comfortable in his own skin.]