There are 65 boys, and the boy ratio is 13: so to find how they got that, divide 65 by 13 and that is 5. Since we now know it is the amount of people, divided by 5, we can take the girl ratio (15) and multiply it by 5 to get the amount of girls (which is 75). You need to find the total amount of students in the class so add the boys and girls (65+75) and get 140 total students