13. (6 points) Two classes in Grade 4 have a total of 72 students, among whom there are 35 girls. Class 4(1) has 36 students, and Class 4(2) has 19 boys. Therefore, Class 4(1) has $\qquad$ girls.
【Analysis】First, subtract the number of students in Class 4(1) from the total number of students in the two classes to find the number of students in Class 4(2). Then, subtract the number of boys in Class 4(2) from the number of students in Class 4(2) to find the number of girls in Class 4(2). Finally, subtract the number of girls in Class 4(2) from the total number of girls, 35, to find the number of girls in Class 4(1).

【Solution】Solution: 35 - (72 - 36 - 19)
$$
\begin{array}{l}
=35 \text { - } 17 \\
=18 \text { (people) }
\end{array}
$$

Answer: There are 18 girls in Class 4(1).
Therefore, the answer is: 18.