So let's make a ratio proportional to this
boys to girls
2/3 <--- the ratio
30/x <---- we need to solve this
let's write as an equation and solve for the amount of girls
2/3 = 30/x
cross multiply
2x = 90
divide both sides by 2
x = 45
The number of girls is 45.
The total number of students in the class is 30 + 45 which is 75.
Hope this helps :)