Okay so you have a ratio of 7:4 so you need to add these two numbers together so 7+4=11.
So now you have 11 and you will need to divide the amount of students by the number we just got(11 in this case)
so 33 divided by 11 is 3.
Now, all you need to do is use your number that you just got (3) and multiply it by the ratio of girls there are.
So you have 7(girls):4(boys) so in this case 7 is the girls so you'd multiply 7 by 3 and you would get 21.
So your answer is 21.