To solve this problem, let's write what we know:
18 students.
1/2 as many girls as boys.
So however many girls they have, there will be twice as many boys.
Let's try it out:
If there were 9 girls, there would be 18 boys, and a total of 27 students. That's too much.
If there were 4 girls, there would be 8 boys, and a total of 12 students. That's not enough.
Can you see a pattern? Try one more.
If there were 6 girls, there would be 12 boys, and a total of 18 students. Perfect!