What was Donald poor at?

Now, a story about a boy whose name was Donald. Donald was not very good at maths. He could not understand the teacher’s explanations. Even when the teacher explained something a second time, Donald still could not understand it.
   "Never mind," Donald told himself. "I’m quite good at other subjects. I’ll sit next to the boy who’s best at maths and copy down his answers when we have the maths exam, then I won’t be in trouble."
   The day of the exam came, and Donald sat next to Brian Smith, who was always at the top of the class in maths.
   Donald carefully copied Brian’s answers on his own exam paper.
   At the end of the exam, the teacher collected the papers and graded them.
   The next day the teacher said to her students, "Well, boys and girls, I’ve decided to give a prize to the student who got the highest grade. It’s difficult for me to decide who to give the prize to, because two students, Donald and Brian, got the same grade."
   "Let them share it," one of the other students said.
   "I’ve thought about that," the teacher said, "but I’ve decided to give the prize to Brian."
   Donald got very angry when he heard this.
   He stood up and said to the teacher, "That’s not fair. I got the same grade as Brian. Why don’t you give me the prize?"
   "It’s true that you’ve got the same prize as Brian," the teacher said. "However, Brian’s answer to Question 18 was I don’t know. Yours was Neither do I."