12. (5 points) 48 students attend a party, the first male student to arrive shakes hands with all the female students, the second male student to arrive only misses shaking hands with one female student, the third male student to arrive only misses shaking hand with 2 female students... the last male student to arrive shakes hands with 9 female students, among these 48 students, there are $\qquad$ female students.
【Analysis】According to the problem, there are more girls than boys, and all the girls arrive early, lining up at the entrance to welcome the boys. The first boy to arrive shakes hands with all the girls and then leads one girl inside; the second boy to arrive does the same as the first boy, shaking hands with all the girls still at the entrance and then leading one girl inside. Similarly, the last boy to arrive shakes hands with the remaining 9 girls and leads one girl inside, leaving 8 girls. It is evident that there are 8 more girls than boys. Using the formula for the sum and difference problem, we can find the number of girls.

【Solution】Solution: According to the problem, there are 8 more girls than boys,
The number of girls is: $(48+8) \div 2$,
$$
\begin{array}{l}
=56 \div 2, \\
=28 \text { (girls); }
\end{array}
$$

Answer: Among the 48 students, there are 28 girls.
The answer is: 28.