3:4 = G:B
1:5 = Mr. Smith's class: 7th Grade
2:7= 7th: Middle school
12 girls = 3 units
1 unit = 12/3= 4
Boys = 4x4= 16
Whole class = 28 students
Class : Grade = 1:5 7 = number of units in Mr. Smith's class
28 = 1 unit
5 units= 28x5= 140 There are 140 kids in the grade
140 = 2 units
1 unit = 140/2= 70
70x7=490
There are 490 students in the whole grade