Is this the topic?
If yes, the answer is as follows:
After a good understanding of the meaning of the question, it is not difficult to find: "There are three more girls than half of the boys."
It means "male students are twice as many as female students, six less", hehe;
After understanding this point, it is added that "the number of boys winning prizes is 3 more than that of girls"
It shows that there are actually 9 girls, 12 boys. Do you understand?
I hope it helps you.