A girl, sitting quietly in her position, holding her chin in her left hand and holding a pen in her right hand, is frowning and seriously looking at the math problem on the table. From time to time, she picked up a pen and counted on the draft book. From time to time, she looked up and stared at the ceiling, as if thinking about something. It turns out that this classmate encountered a difficult problem when doing his homework, and he is seriously thinking about it! Look, how focused her expression is! Her eyes were fixed on the exercise. For a long time, a happy expression appeared on her face. She must have solved the problem.