Ok. Well the way i see it is that since there are 12 boys in the choir, and that is 3/7 amount of the class. Then you would divide 12 by 3/7 which equals 28. So i believe there are 28 kids in the whole class. I think to check, divide 3 and 7 which would equal 0.428. Then divide 12 and 28 which would equal 0.428. So they are the same amount.