A number for a particular ratio can be calculated as: Let x= total number of students Ratio of boys : girls = 3:2 Ratio for girls =2 Sum of ratio =3+2=5 Number of girls is equal to the ratio of girls over the sum of the ratios times the total number of students Every number is being divided by 1 420=420∕1 Rewrite the equation Cross multiply to solve for the value of x 2x×1=5×420 Divide both sides by 2 Cancel out x=5×210=1050 Total number of students =1050 ALTERNATIVELY The ratio for the total students is the sum of the ratios Ratio for the total students =3+2=5 Let x= total students We know the ratio of girls and the number of girls Equate the ratio to the known numbers Ratio of girls = Number of girls Total ratios =x (Total number of students) Cross multiply to solve for the value of x 2×x=5×420 2x=5×420 Divide both sides by 2 Cancel out x=5×210=1050 Total number of students =1050