12. (3 points) Xiao Xi bought a total of 30 exercise books of type A and type B, paying 54 yuan, and got 0.5 yuan back. Type A exercise books cost 1.5 yuan each. Type B exercise books cost 2 yuan each. How many type A exercise books did Xiao Xi buy? $\qquad$ books.
【Solution】Solution: $(54-0.5-1.5 \times 30) \div(2-1.5)$,
$$
\begin{array}{l}
=(54-0.5-45) \div 0.5, \\
=8.5 \div 0.5, \\
=17 \text { (books), } \\
30-17=13 \text { (books). }
\end{array}
$$

Answer: Type A exercise books bought 13 books.
The answer is: 13.