11. (6 points) Xiaohong bought 1 pen and 3 notebooks for a total of 36.45 yuan, where the price of each notebook is $\frac{15}{4}$ times the price of each pen. What is the price of 1 pen? $\qquad$ yuan.
【Solution】Solve: $36.45 \div\left(3+\frac{15}{4}\right)$
$$
\begin{array}{l}
=36.45 \div \frac{27}{4} \\
=5.4 \\
5.4 \times \frac{15}{4}=20.25 \text { (yuan) }
\end{array}
$$

Answer: The selling price of 1 pen is 20.25 yuan.
Therefore, the answer is: 20.25.