In the class, there are 30 students: excellent students, C-grade students, and D-grade students. Excellent students always answer questions correctly, D-grade students always make mistakes, and C-grade students answer the questions given to them strictly in turn, alternating between correct and incorrect answers. All students were asked three questions: "Are you an excellent student?", "Are you a C-grade student?", and "Are you a D-grade student?". 19 students answered "Yes" to the first question, 12 to the second, and 9 to the third. How many C-grade students are there in this class?
Let $a$ be the number of excellent students, $b$ be the number of poor students, and $c$ be the number of average students who made a mistake on the first question, answered the second question correctly, and made a mistake on the third question.

According to the conditions, $a+b+c=19, b+c=12, c=9$. Therefore, $b=3, a=7$. Thus, the number of average students is $30-7-3=20$.

## Answer

20 average students.