11. (12 points) The library purchased 300 books of 5 types: "Zhuangzi", "Confucius", "Mencius", "Laozi", and "Sunzi" for 4500 yuan. Their unit prices (the price of one book) are 10 yuan, 20 yuan, 15 yuan, 30 yuan, and 12 yuan, respectively. The number of "Zhuangzi" and "Confucius" books is the same, and the number of "Sunzi" books is 15 more than 4 times the number of "Laozi" books. The number of "Sunzi" books in this batch is $\qquad$ books.
【Solution】Solution: The average price per book is:
$$
4500 \div 300=15 \text { (yuan) }
$$

Since the number of copies of "Zhuangzi" and "Confucius" are the same, $(10+20) \div 2=15$ (yuan),
we can assume that the price of "Zhuangzi" and "Confucius" is 15 yuan, and the original price of "Mencius" is also 15 yuan, so as long as the average price of "Laozi" and "Sunzi" is 15 yuan;
$$
\begin{aligned}
\{15 \times & (15-12) \div[(30-15)-(15-12) \times 4]\} \times 4+15 \\
& =\{45 \div[15-12]\} \times 4+15 \\
& =15 \times 4+15 \\
& =60+15 \\
& =75 \text { (copies) }
\end{aligned}
$$

Answer: There are 75 copies of "Sunzi" in this batch of books.
Therefore, the answer is: 75.