Why can't I do this?

Gabby opened her math book and put Nara on her lap, "Look at this problem."

"For the first problem, determine the determinant of the following matrix..." Gabby took out her blank paper and started counting.

Five minutes later.

"Well, do you understand?" Gabby looked at Nara's face. 

When Gabby saw Nara's almost closed eyes, she stroked her back then sighed, "How could you want to sleep now?"

Not long after, Agnes opened the door to Gabby's room. When she saw her daughter was trying to wake Nara, Agnes couldn't stop laughing.

Gabby looked up and closed her book, "What's wrong, mom?"

Agnes put down a small plate of fruit and stroked Gabby's hair, "Still studying, huh?"

"This math assignment needs to be submitted tomorrow." Gabby took the grapes and put them in her mouth, "Not collected, maybe tomorrow we have to work on the front board."