we put the ratio between girls and boys then we add the (total) or(sum) beside them in the same ratio then the summation of the ratio, like (5 +4 = 9) and then under the word "boys" we put the number given in the question which is (12) , now we need to know the number of girls in the class so we use the cross multiplication which i wrote at first.
by that, you can calculate no. of girls and no. of students in the whole class if you want.
